I'm really not a fan of doing in-game achievements. Medals should be as far as it goes, because that's all local. Even then, all of this external client app stuff is on the back burner for now. It's all insignificant for the development of the stats tracking.

Like FAB said, if you do achievements, it requires constant tracking of every single person. Doing this with Xbox Live is OK, because they built it over the course of 2-3 years. Also, with Xbox Live, a lot of achievements are locally calculated (such as overkill), then communicated back over the encrypted network, communicated back, and then they are displayed on your screen. Doing the same thing with a game that pre-dates Xbox Live is way different.