I thought the main quest could have been better, the fact that time on the main quest stood still while I farted around for days on end bothered me... things should have progressed with or without me - with my character participating tipping the balance. You should have been able to ignore the main quest if you wanted to but then a few of the cities would fall.. but if you did what the main quest wanted, you would save everyone.
There should have been signs of an army gathering, recruiters in the cities taking people by force if need be. Progress being made. Instead, everyone was waiting on me to do things... even the bad guys.
And about the bad guys... the planes of Oblivion, creatures lived there? Really? They felt like large quake maps. The place was a mess, no wonder the bad guys wanted to come thought.. their place was all busted up.
Hopefully, Skyrim fixes this by weaving the main quest into the game world a little better instead of it being a tacked on - when you've got time, if it's not too much trouble... kind of thing.