I recently found that the AI you picked for your custom game can make a huge difference in gameplay.
Being a Gal Civ 1 fan, the first two GC2 games I played I've always chosen the "Arceans", "Yor", "Drengin", "Torian", and "Altarians", that is, the 5 classic races {I am the Human). In both of those games, I had great fun, doing a little of research, developing, expansion, influence, and war. You can't beat games like that without involving yourself in all aspects of the game.
But a few days ago I began to give the new races a try. I played as the Torians against "Drath", "Iconians", "Korx", "Thalians", and "Human". The game is still fun, but it's a lot less exciting than the last two games I've played in which I used the classic races. I pretty much beat the whole game without researching a single military tech or building any combat ships. My influence just keeps spreading and the AI doesn't do a thing about it. It certainly is a very peaceful galaxy!
By the way, I played all games in the "Normal" AI setting.
This really leads me to wonder if there is an ideal combination of races that would offer the most fun gameplay?