The first two instances are very good. The difficulty caught me by surprise and my first group wiped on only the third pull, as did my second group. The instances are small but challenging requiring you to know what to do in advance (so basically you learn the hard way

). Even the early instance bosses have some vicious DoTs which require immediate dispelling or it's certain death. The quest rewards are well worth it for your non epic players including the random drops. The boss loot tables are very nice.
The instances are linear which is sooo nice after instances like Gnomeregan, Sunken Temple and BRD when you can get totally lost and have no idea where your quests are. This is not to their detriment though, you can simply concentrate on what you went there to do. In this sense they seem to follow the ethos that the outdoor experience gives, it's straightforward but interesting, it's not a chore, it's quick if you know what to do.
Annoyingly but unavoidably you do still need the 'holy trinity' of a tank, healer and dps, so people still insist on a priest (regardless of spec) and a warrior (regardless of spec). DPS can come from the usual sources but you still don't
need a rogue, hunter, paladin, druid, warlock or mage though obviously they each bring something unique to the group. However I think I'll die the day I see "LF1M Hunter" for a 5 man instance
I also like that fact that getting the instance quests doesn't require you having to travel across the world to talk to one person in a cave full of elites 10 levels higher than you that you'd never find under normal circumstances, they all simply follow on from other quests in that zone.
They've also made the outdoor bosses, and by that I mean the one's at the end of normal quests (not raid bosses, just standard elites) one or two man jobs. This makes finding a group very easy. Hell I was able to solo 63 gold elites as a 62 ice mage
Like I said though, not seen the higher level content yet, it could all change...