How to unlock heroics

Since BC already we have heroics in the game. Heroics are a more difficult version of an instance, and generally give better rewards. Since Wrath we have heroic raids as well and hard modes, and then of course there are still achievements so you can do a boss in a certain way to get the better loot.

In Burning Crusade there was a uniform system to get into heroic dungeons. You had to become revered with the specific reputation that went with a dungeon and then you could do this dungeon on heroic mode. But in WotLK the unlocking of heroic modes has become diversified and isn't always as clear as it could be.

After the break an overview of how you unlock the different heroic modes in WotLK.

Dungeons

Normal mode dungeons become available throughout the leveling process. WoWWiki has an excellent page of the level requirements for each instance here, but I'll give a quic overview of the northrend dungeons below. Every dungeon becomes available on heroic mode once you reach level 80.

• Numbers indicate the minimum level to enter, if known; and the recommended level range (in parentheses).
• The recommended level range is also the range required to be able to be summoned via the summoning stone.
• Note that the level range required to use the Dungeon Finder may differ from the recommended level range.

• Utgarde Keep 65 (70-72)
• The Nexus  66 (71-73)
• Azjol Nerub 67 (72-74)
• Ahn'kahet: The Old Kingdom 68 (73-75)
• Drak'Tharon Keep 69 (74-76)
• The Violet Hold 70 (75-77)
• Gundrak 71 (76-78)
• Halls of Stone 71 (77-79)
• Halls of Lightning 75 (80)
• The Occulus 75 (80)
• Caverns of Time: Culling of Stratholme 75 (80)
• Utgarde Pinnacle 75 (80)
• Trial of the Champion 75 (80)
• Forge of Souls 75 (80)
• Pit of Saron 75 (80)
• Halls of Reflection 75 (80+)

Raids

Naxxramas

This instance is available at level 80, and offers options to get achievements if you kill bosses in a certain way. Naxx never had hard mode loot however, and while there was a dragon to be earned in the very beginning by completing all the achievements, now the only reason you would still do this is for the pretty ding at the bottom of your screen. (wow, another achievement)

The drake was removed the moment Ulduar was introduced. 

Malygos

Malygos only has achievements, no hard modes.

Sartharion

The more drakes you leave alive while fighting the big dragon, the better your loot becomes. When you kill him with all three drakes alive you've earned yourself a dragon.

Ulduar

This instance is available at level 80, and next to achievements, and a drake for those achievements, you can earn hard mode loot by killing bosses in a specific way. Not all bosses have a hard mode in Ulduar.

Flame Leviathan - Speaking to the dwarf starts the fight on normal, but when you speak to the keeper first, the fight can be done on 4 different modes depending on how many towers you leave up. The more towers you leave up, the harder the fight becomes, and the better the loot.

XT002 - Not killing the heart means you're doing the fight normal mode. When managing to kill the heart the fight switches to hard mode.

Iron Council - The order in which you kill these makes that you do the fight in hard mode or not. Killing big > middle > little guy is considered normal mode, killing the middle > little > big guy is officially the hard mode and will earn you the extra loot. However, the hardest mode is considered killing the middle guy last, simply because of the design of the encounter when doing it like this. 

Hodir - Kill him before the extra chest disappears and you have the hard mode loot in your pocket.

Thorim - Get through the hallway fast enough to make sure the blue lady sticks around and you enter hard mode

Freya - The more trees you leave alive so they can help Freya to fight against you, the better your loot becomes.

Mimiron - See that big red button that says, "Do Not Push This Button!"? Yeah, push the button and hard mode begins.

Vezax - Do not kill the vapours and you get the saronite animus, kill him and Vezax, and you beat him hard mode.

Yogg - The less keepers you bring down to help you, the harder the fight gets, and the better the loot. 1 keeper is considered the hard mode, and 0 keepers is currently considered impossible..but hey, feel free to try. 

Trial of the Crusade

With this instance another new way of doing hard modes was introduced. Now you can decide on your character portrait whether you want to do the entire instance on normal or hard mode.

No longer do you have to relearn the dance. Same dance, just harder. There is only one trick to it, you have to have done all the fights on normal mode before you can switch to hard mode.

Icecrown Citadel

The final raid instance of Wrath of the Lich King. This instance uses the same mechanic as Trial of the Crusade, but now you can decide boss per boss if you want to do them hard mode or not.

No hard modes have been unlocked for this instance yet, since here too you have to have finished the entire instance before hard modes are unlocked...and yeah, Arthas is not available yet, so no hard modes yet.