If I'm active and do not have the distraction of other worlds, I actually love start-ups. I consider myself quite decent at start-ups, though I tend to try to start away from the core. I started there in W49 and failed miserably. Starting closer to the rim allows you to pay less attention than you would have needed to if you were in the core; less chance of someone clearing you while you sleep.
Mid game can be seen as the most enjoyable, especially for the diplomacy side. Wars don't usually mean too much early game, and wars last longer in late game, but in mid-game, diplomacy is constantly changing, pnp is most active and there is a genuine interest in how to play. Playing-wise, you could say it is the jack of all trades but the master of none.
End game has its highs and lows. I love the fact that you have lots of troops to play with, but end game is all about trying to make the other player quit, rather than rim him. This means that if you are targeted, you are hit with several thousand incomings. Faking becomes much more of a priority in late game. Stacking can become kinda ridiculous and takes away from the aspect of defending, but it works in a lot of cases, so why not use it? There is also the monotony of it all. Coming up to two years playing the same game almost every bloomin' day. The fun starts to turn into obsession to win. In order to "win", you may sometimes have to accept that tagging your incomings, dodging, sniping, etc, for days/weeks at a time, leaving you with several sleepless nights. Eventually, like Slowdowntubby did, it will just leave you thinking, "Why am I killing myself over something so arduous and repetitive without any sort of reward whatsoever?", leading you to quit the game.