I think a game needs four things to be fun for a long time. Two have to be in the game and two can come later:
Ingame:
1. Balanced Options: Many different paths to go or few but really distinctive ones. This may be realised by unit diversity or the tech tree or something else. The problem is to balance these. There are quite good games that have alot of options like C&C Generals. You can choose several distinctive abilities with the strategy points. Problem is that I usually choose the same as they are just too powerful.
Sins has some interesting options with the capital ships and the tech tree. They could be more distinctive though but the games is not finished yet and the other two races will bring more options.
Nothing to say about balance before Beta 4.
2. Opponents: More balanced options, more fun in multiplayer. That is a simple formula. But there are alot of people who can't or don't want to play online, so good AI's that don't always do the same thing are key, too. Games with different AI profiles (Battle Realms, Civilisation) give you a new challange in every game for a long time. In both games the AI can also be scaled beyond the easy/medium/hard scale. Battle Realms has a handicap system for this which is also useable in MP.
Sins AI is not finished yet. I really hope the AI will be good, because it would be a pain to play 10 player games online for example

Outside the game:
3. Mods: Good editors and modding support keep games alive. Neverwinter Nights 1 is a good example for it. Most normal casual gamers can do marvellous things with the editors. So far IC seems on the right track.
4. Support: This means not only patches for problems and balance issues but also new material once in a while. Ranging from a new map to add-ons. Stardock's GC II is a good example for it