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.
First I played Hattrick to win, but over the years, I came to realize that reaching the absolute top wasn't going to happen for me - I'd need to spend too much time to fully understand the game and make use of that knowledge. So I stuck around because of the great atmosphere and people in the forums. Recently my interest in the game itself increased again, which made me open a team in a new league (Botswana), and I'm trying to be at least semi-competitive there.
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!
I play Hattrick because I am a big football fan and I have always liked to play football manager games, like the German game "Anstoss" or "EA Football Manager". I play Hattrick since 2002, even though I had a break because of the birth of my daughter. So Hattrick has been with me for most of my life, you could almost call me "addicted” to Hattrick!
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.
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.