My feedback on why it hasn't grabbed me yet:
1) Design - as Goronmon pointed out above, the design doesn't feel fun or encouraging.
2) Performance - it's laggy right now, although I expect that will improve soon.
3) User Experience - It's hard to figure out how to get started, or to get a clear picture of how it all works. I'd suggest including lists of activities, rather than making people type into a blank box and look for autocompletes.
The activity feed is interesting, but it still feels like a list of data, removed from any context.
4) Perception - Right now, it seems like a place for fitness freaks and gym rats. That might be intentional - but I'd much rather use a site populated by out of shape geeks working to get fit.
If Fitocracy can make exercise game-like and enjoyable, it will win. The hardest part is getting started each day (getting up to run, going to the gym instead of the pub after work) - that's where Fitocracy can make a difference.
Good luck, guys - I hope this becomes something great!