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.
One reason I play Hattrick is because I like the cyclical nature of the game. You start with an idea about what kind of team you want to build, then you train long-term (up to 3 years in real life!). While you build, you can test and try to comprehend the aspect of the match simulation that you want to master. I also like the tactical choices you have to make for each match. But the most important component is the community.
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.
What I like about the Hattrick community? Everything. I like all the help you can get in the forums about the game, but also for everything else it gives you in everyday life. Hattrick is my favorite social network.
The Hattrick community is colourful, it really helps expand the Hattrick world. You can find plenty of helpful managers that can help you with Hattrick and real life questions.
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.