What I like about the Hattrick community is that there are real people and you don't just compete against the computers. You play against each other, but you can also help each other in the forum and talk about everything.
My biggest achievement in Hattrick so far is that I have won some titles with my second team in Bahrain and reached the semifinals of the National Cup in Italy, but I hope, a few months from now, that it will be winning the World Cup for the Italian NT.
The game concept I like the most is the Special Events system, which allows multiple tactical options that are almost equal in strength. In general I think the Youth Academy is the best feature in the game as it creates a close connection to your own players.
I like that Hattrick is a slow game. Here, I have time to think everything over. I also appreciate that Hattrick gives everybody the chance to be the best because you can't buy in-game advantages.
One of the best parts of Hattrick, for me, is the fact that you can somehow massively relate to a bunch of fictional players with fictional names and treat them like they're real world players.