Some notes, starting in TF2 and wandering into CSS...
1) The TF2 servers are a bit more "strict" about rules. Not to say that other divisions are lax, but the TF2 community is pretty tight about cussing, griping, griefing, etc. I think it's because the TF2 players are less entrenched in their ways than CSS players. My take.
2) TF2 the game is full of variety. I played Engie until I was really good and then switched to Medic. It's like a whole different game as Medic. Same for Soldier and Heavy. Each class takes it's own cycle to get "good," making the game last that much longer.
3) The eGO TF2 servers are so well known I've gone into an empty server and had it fill around me at 2AM Central. If the choice is between a regular TF2 server with lots of people, and an eGO server with two or three, I'm surprised how many pubs pick eGO

Or... maybe I'm not surprised...
4) Because of the decent churn in users on TF2, and because the gameplay of TF2 really requires teamwork (you can win on CSS by being super pwntastic all by yourself with a team of redshirts), that strategies that worked great one hour ago need to be modified as the players churn between teams and servers. So the battlefield is constantly changing...
5) As an extension, the type of people who join TF2 servers are already team-minded... they come in expecting to fit into a group. So they're less likely to cause trouble or get out of line. I've seen situations that lead to bans in CSS that wouldn't get past the "it's the rules" stage in TF2, just because TF2 gets you in the mood to play along
6) TF2 is just fun. In CSS, every character class and weapon is pretty much the same: point and click. Some weapons have zoom, some are grenades, but really, it's point and click. And other than vanity, there's no reason to pick on character class over another. TF2 lets you pick PLAYSTYLE through the character class selection. Do you want to waddle in and toss death at everyone you see tonight? Heavy. Do you really just need to see people explode into humorous itty bitty bits? Soldier or Demo. Do you want to case people around as they run in terror? Pyro. Want to sneak around and trick people into dying for you? Spy. Interested in helping the team more than being the hero yourself? Engie or Medic. Want to pick people off from miles away? Sniper. And when you get tired of lighting heavies on fire, new excitement is just a classswitch away. (I made that a compound word just because I wanted to see 3 s's in a row.)
Insurgency is fun, but I burn out in an hour or so. CSS is fun, but I burn out in an hour or so. I've lost WEEKENDS to TF2
