There are 3 things I look for in an alliance.
1) A community - I need an active and vibrant community that will both stimulate and entertain me. If an alliance isn't a community, or doesn't have enough active members in it, you will soon get bored. The fellow alliance members are probably the most important aspect of the game for me.
2) Loyalty - If you demonstrate your loyalty to the alliance, the alliance should demonstrate it's loyalty back. An alliance should value each and every member equally, and be just as willing to die for it's smallest nation as it is for it's leaders.
3) Free Speech - I have a right to express my beliefs on anything that people decide to bring to this public forum, and anything on the private forums of my alliance. If I maintain a level of respect towards those I'm disagreeing with I don't expect my alliance to ask me to be quiet, even if my views go against the majority.
If you can find those 3 things, I think you'll end up enjoying being in an alliance more than you did being alone.