When the A.I is building the victory condition the player will basically stop it.The A.I factions need to be proactive in actually stopping you winning.It just means you end up doing nothing for 20+ turns to win.Makes the end game boring.
I think this is the biggest problem with the end-game, and the easiest way to improve it. My evidence for this is that in MP, you really can't just build your wonder and wait to win -- the other players WILL try to stop you.
There are some domino effects to that. Because you know you will have to defend your wonder, you can't just rush to victory and ignore military/play as tall as you want. You may not even want to start building your wonder right away once you can, since you'll need to make sure you have a big enough army that you can stop or at least slow the other players from getting to it. So you end up actually using a lot of end-game techs, including the ultimate units, on your way to victory.
The handful of MP games I've played to completion against players that know the game well have been way, way more interesting than SP. Actually in one Soyuz game, me and my two human opponents spent so much time killing each others' wonders (went back and forth a good 2-3 times) that the PAC AI eventually finished the Mindflower and won before anyone could stop her.