GTAIV - my graphics card is just a bit too old. It runs everything else fine... I see other people regret playing it, but I would have liked to have got even that far.
Sim City 4 - Each city zone feels too small, and the game completely lost the charm of sc3k. And instead of fixing the problems of sc4, they then bring out Societies, which misses the whole point of Sim City. I still think Sim City 3000 is the most important game anyone can play if they want to get into politics...
Red Alert 3 - It should work. It really should. But it fell really flat, in my opinion. I mean, it's Red Alert, so it's supposed to be an high camp rpg, but the humour was contrived, and the 'co-op' single player idea was a total clusterfuck in that it was completely unnecessary, as the opponents were never difficult enough that it required an assistant. Plus the special abilities were too fiddly to use properly.