I picked it up. Hit 70 three and a half days after I got it (granted, on very little sleep).
For the most part, most pieces of t3 won't be replaced until the latter stages of Karazhan (when t4 starts dropping). So while the reports of level 61 greens replacing some epics is very true (particularly with ZG, RAQ and t1 epics), most raiders won't replace their gear until the late 60's or the beginnings of the 70 end-game.
I was in beta, so I already have a lot of experience with the expansion. To describe some of it:
There's still reputation grinds, but they don't take nearly as long and the rewards they offer don't require stupid shit like
insignias to obtain. Once you hit a certain level of reputation, you can just buy the rewards with gold. Most of the reputations can also be ground in instances.
All of the new 5-man instances have two versions: a normal version, and a heroic version. Some instances are built for characters that are below 70, when on normal mode. On heroic mode, these instances change in difficulty and loot to be suited for level 70 characters. Dungeons that are built for level 70's on normal become more difficult and also have better gear when put on heroic mode. All dungeons on heroic mode drop tokens which can be used to purchase
some epic items.
The zone design in the expansion is a lot more clean than it was in the original game. Objectives for various quests tend to be clustered together, instead of spread out across the zone. Some of the quests are also more interesting.
Top-level raiding is now built around 25 characters, instead of 40. And mid-level raiding is now built around 10 characters, instead of 20. This makes raiding a little more accessible, but has destroyed many guilds that were built around raiding with 40 people.
Blizzard has implented a new type of PvP: arenas. Arenas can be 2v2, 3v3 or 5v5. You're also able to set up arena teams (for 2v2, 3v3 and 5v5). You can be in both a 2v2 and 3v3 team, but you cannot be in two 2v2 teams. An arena team can have twice the number of people required for the match on its roster (i.e., a 5v5 team can have 10 people rostered). Each arena team gets its own channel (like a guild channel). For rated games, you must only play with people from the teams you're on (i.e., you must play rated games of 5v5 with people from your 5v5 arena team). Rated games can change your team's rating in the same manner that a game of chess can change a chess player's rating. At the end of each week, arena teams will obtain a certain amount of "arena points" based on their rating at the end of that week (a team must play 10 games in a week to receive those points, however). There are also seasons, in which top ranked teams will receive various rewards at the end (one of which is an epic flying mount). At the end of a season, ratings are reset. Seasons will last several weeks.... As a note, you can play non-rated arena games with anyone (no teams required). Also, arena games can be AvA, HvH or AvH.
As far as the new races go, there's not much to it. Draenei priests get fear ward, so I imagine the Horde will complain about that quite a lot (justifiably). The starting zones for both new races are great. The first blood elf zone looks like nothing else in the game, with bright yellows and reds. The second blood elf zone is in sharp contrast to it (being plagued and all). I would say it looks similar to the plaguelands, but it has striking difference. The first draenei zone, meanwhile, looks more like Darkshore (except more blue-green in color). The way its structure is a lot better, however, and there's actually an interesting line of quests in both of their zones.