I gotta give a nod to Graylo over at Gray Matter because he's already talked about this and we don't have drastically differing opinions. That said, I'm going to be putting forth what I think makes a good Heroic Encounter.

But before I do that, let's mince words, shall we?

Heroic - Impressive in size or scope; grand; Of a size or scale that is larger than life.
Hard - Requiring great effort or endurance; Difficult to resolve, accomplish, or finish; Intense in force or degree.

I find it difficult to distinguish between the two things in WoW. Perhaps Blizzard intended to have both Heroic and Hard modes for bosses at one time. Now I feel that the two have become quite synonymous. That said, I'm going with my gut and saying that Heroic and Hard mean the same thing in WoW: An encounter that requires greater effort to complete than its non Heroic/Hard counterpart. Notice that this definition mines itself from Hard not Heroic. To me, a Heroic encounter would be identical to the normal encounter only ... bigger. That's pretty uninteresting.

Abilities

Ramping up abilities or having their overlap increase is a cheap gimmick. Yes, it can totally change the dynamic of the fight, but generally it leads to a fight that requires you to juggle the same number of balls in roughly the same way, but perhaps faster. Most of these encounters can be "outgeared" to the point where the normal and heroic versions don't seem too drastically different.

The introduction of new abilities is generally an easy way to bump up the difficulty. Let's take a look at some of the heroic versions that do this. First, there's FL in Ulduar. The abilities used during the fight come directly from player choice. I really liked this idea and especially enjoyed the way loot was handled in the fight. The more difficult the player elected to make the fight, the more loot they would receive if victorious. Unfortunately, there wasn't an On/Off switch. There was no way to change your mind after you started. I don't see the need for such firm commitment to a heroic mode.

This "model" was done with greater success at the Iron Assembly. Regardless of your kill order in previous attempts, there was absolutely no limitations on your kill order in the following attempts. As the fight progressed, each remaining member of the Assembly gained a new ability. Your kill order determined the abilities you encountered and the difficulty of the fight.

Freya offered a FL-esque type choice. Her special abilities came from her remaining guardians. Kill a guardian and it was dead. Irreversible decision. Regardless, Freya offered many choices and many paths to success. This type of player choice creates what I'm going to call a "rich" encounter. The various difficulties can be chosen to play to the raid's strengths. It also allows an encounter to scale, offering raids several authentic and new encounters instead of a ramped up version that only requires more damage and healing throughput.

Implementation

Selecting "Heroic Mode" from a menu is pretty damn lame. While it does allow a raid to reverse its decision quickly, it seems so incredibly lackluster. Where's my big red button?! Where's the DPS check to even activate the hard mode? Where are the mobs assisting the boss that I can either take out first or leave up for a more difficult encounter?

In my opinion, Heroic Mode should be put down. Decisions about how to engage a boss should involve dealing with its subordinates, kill order, pressing an awesome red button, or some other nifty mechanism that fundamentally alters the encounter. This adds a great deal of logic to the fact that the boss now has abilities it did not have before.

Loot

Currently, Heroic Modes offer a higher ilevel gear. I realize that this carrot is probably a must when it comes to voluntarily increasing the difficulty of a fight. Sarth did a good job with managing the loot. Increasing the difficulty offered a chance at slightly higher ilevel gear. Blizzard made it clear that they wanted to do away with loot table bloat, so now we have duplicate items that are scaled up. While I appreciate the desire to have a streamlined loot table, I have to wonder if it is really that much harder to have loot that is only accessible through the Heroic Mode. There was a point in time when you could look at a player's gear and just from the graphics have an idea of their progression. Tier gear has followed this pattern, but is now accessible through regular 5 mans with enough dedication. It would be nice to see some distinguishing gear instead of a distinguishing mount that you can't even ride in the main city (without taking advantage of some glitches).

One easy way to set gear apart is with weapons. Having weapons in the game that only fall from the Heroic Mode will provide exactly the visual recognition that I think should be associated with conquering the top heights of WoW. Even now,

We were given some license this week to choose the instance we want to spruce up. I picked Ragefire Chasm (map) because it's an instance that I used to ghetto-hearth all the time and it never got the respect it deserved from me. Also, it's massively accessible in much the same way the Stockades are massively accessible to the Alliance.



Here's a little back-history from Wowikki.com

Ragefire Chasm was first discovered by Chief Goblin Engineer Gazlowe, during Rexxar's visit to Orgrimmar. Beneath the city of Orgrimmar lies this fiery chasm. Recently, a clan of troggs known as Ragefire Troggs have begun to appear in the chasm, from deep below the lava-filled tunnels. Magatha the Seer, of the tauren people, attempted to make peace with these creatures, but was met only with hostility. It was clear that these troggs were not interested in diplomacy, and could potentially overwhelm the entire Horde if left unchecked.
A sect of the mysterious Shadow Council has also taken refuge in this chasm. This sect is known as the Searing Blade, and is led by Taragaman the Hungerer, a felguard; Jergosh the Invoker, a powerful warlock; and Bazzalan, a satyr. From their dark hiding place, they plot to overthrow and destroy all that the Horde has built for themselves in these lands.

If you haven't run RFC, you should. The very first thing to do in RFC is go find the corpse of a Maur Grimtotem. His earthly remains are guarded by his assumed assailant Oggleflint. For shame, Maur. Who gets killed by someone named Oggleflint?!?

From there, you mosey on over to kill a felguard named Taragaman the Hungerer. What a great name. Next in line is an Orc Warlock named Jergosh the Invoker. He's a leader of the Searing Blade and prototypical angry/demonic orc. He gives orcs a bad name! (yes, you should be singing Bon Jovi) Finally, you trek up to Bazzalan, a Satyr who also acts as a leader of the Searing Blade. All in all a fun time with several different feeling bosses. Good trash packs too!

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And then, the mountains were sundered and the lava pools below burst forth, also allowing the release of some very dark magics planted in the deepest bowels of the Ragefire Chasm by none other than the nefarious warlock Jergosh. In the midst of violent magics and tremendous forces, the Ragefire Chasm thrust through the crust of Azeroth and pushed towards the sky, bringing with it the remains of those killed previously.

Jergosh the Invoker returns as Jergosh the Cursed, an undead warlock of immense power. He summons to his side the Satyr Bazzalan, rotted and decayed but held together by powerful dark forces, this Satyr knows no remorse or mercy and will gladly sacrifice parts of himself to smite his enemies.

Unintentionally resurrected in the chaos was the brave spirit of Maur Grimtotem. His spirit ripped from the serenity of the afterlife, Jergosh quickly realized his power and imprisoned him in a chamber deep inside the Ragefire Chasm. Maur is in grave danger of being consumed by the dark forces swirling in the ever-rising lava, where his soul would be destroyed and lost forever from the light.

Adventurer's are moved by the plea of Magatha Grimtotem, who sensed her clansman's spiritual return and torment. Magatha places a bounty on Jergosh's head and a reward for the safe return of clansman's soul.

10 Changes:

1. Rising Magma - This is the biggest change. Much like Molten Core, you would be working your way down into the heart of Azeroth to free Maur of his confines. The magma seeping into the Chasm will destroy his prison and his soul in 25 minutes after you enter the instance. Possible Achievement or special mount here.
   
   
2. Rescuing Maur is a new Daily Heroic Dungeon quest.
   
   
3. Jergosh the Invoker becomes Jergosh the Cursed, twisted by his own evil magic. He no longer casts curse of weakness but rather has a new aura, reducing all attributes by 3% every 5 seconds. Jergosh learned Conflagrate to accompany his vicious Immolate. Conflagrate hits for a large (>15k) amount of damage. He will also summon souls of those that have died in Ragefire to attack as well.
   
   
4. Achievement for killing Jergosh in under a minute.
   
   
5. Bazzalan will have targetable parts much like Kologarn (only smaller in scale) and each different part, when destroyed, provides Bazzalan with a new ability as he integrates the destroyed part into his arsenal.
   
   
6. Possible Hard Modes on Bazzalan depending on the kill order of the parts.
   
   
7. Taragaman the Hungerer is resummoned by Jergosh upon the first encounter with Jergosh. Jergosh teleports himself to the gate of Maur's Prison after summoning. Fire Nova range increased to 100 yards to force damage on group. Taragaman also has an intercept and an enrage/focus for 10 seconds every 30 seconds. During this time he is untauntable and must either be kited or healed through.
   
   
8. Trash packs WILL require CC until massively outgeared. Many of the humanoids will have AoE capabilities and need to be controlled.
   
   
9. Troggs seem to be thriving on the demonic energies and have become much more powerful and aggressive.
   
   
10. The Lava Elementals have fused with the Earthborers to make what appear at first site to be worms made of Lava. Due to their fiery hot bodies, they cause durability loss on weapons at a slightly accelerated pace.

That's my quick list. I'd really like to see Ragefire Chasm rise up from the ruins of Durotar if it does get destroyed like I heard whispers about. So, adventurer, what dost thou think?