While I do not have much experience in the level 60 scene I know quite a few people who have stuck with their 60s since BC was released, since Cataclysm was released a lot of the old raids have been removed so most of them have decided to level to 70. Also at level 60 you almost never get BG queues, and when you do they are few and far between (from what I've heard from them). So must have just givin up on 60 and leveled to 70 because of how popular it is now.
The good thing about level 70 is it's all based in Outlands, where NO raids have been removed/changed in any severe way that will decrease your fun. As for PvP the gear is so extremely easy to get now that a lot of people who weren't even into twinking have started doing it just for the pure fun and ease of it, this can be a drawback and a benefit. Most ques pop almost quicker than level 85 BGs.
In regards to 70 twinking in general and why it's so fun and popular is you get almost ALL of your base spells/abilities at that level, you have your damage spells you have your best heals and you have almost all of your utility. I would guess that you train 90% or more of your spells/abilities by level 70 so you actually feel like your class/spec. Also at 70 currently a properly geared and competent player can play just about any class/spec combination effectively there is no TRUE OP class or spec as long as you geare well and know what you're doing you stand a good chance 1v1 vs almost anyone, which is extremely hard to say for any other level cap including 85. You just feel the balance at 70, and the core group of players you run into will generally be easy going and fun to play with.
GL on your choice, looking forward to seeing you in a BG at 70 someday soon