I would say the community is the strongest reason I have stayed in Hattrick for so long. The game too of course. After having trained my players and played in the French Division 2 for a while, I started focusing on training international players to help the Moroccan team. Today 80% of the defense of the Atlas Lions was trained by me!
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.
I am one of the old users who likes to do my own research to learn how the game works. I like that this is a thing in Hattrick. Not everything should be mentioned in the rules. I also like the federations where you can have some real tight friendships. More than some friends call Hattrick a forum, with a soccer-game attached to it.
I ended up playing Hattrick because I used to play some MMORPG in the past (like Dark Age of Camelot or Ultima Online) but they were totally absorbing and some of my friends described Hattrick as "easy and casual", and I believed it (lol). I couldn’t really guess, at that time, how much time and passion I would end up putting into this game.
What I like about Hattrick is that there are not 248 925 784 skills that you have to pay attention to. I like that there are several (but not too many) tactics and that they all have weak points that other strategies can beat. I like that this is a slow game, it fits my private life. And that is probably also why I still find it interesting after almost fifteen years.
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.