I've been on several other gaming forums, but I haven't stuck with any of them, because none besides GAF has this mix of qualities:
1. Large enough to have decent game discussion but small enough to not feel crowded
2. Absence of politics; no polarization/alienation of members along party lines
3. Multi-platform site, not heavily skewed to one console or another
4. Good sense of humor
5. Light moderation
6. Diversity of opinion
It's hard to find that mix anywhere else. The only downside is the fanboy types, but you get them everywhere.