Cons:

1. No gaming-If you are a serious gamer, this OS is useless for you.
2. No shockwave-You cannot probably play much games on the internet because they use shockwave flash, and as such there is no plugin available for that in linux for the time being.
3. Inferior drivers-Some things go wrong sometimes with the drivers in Ubuntu. When listening to music on high volume, the sound often bursts, whereas i get much better sound at the same volume in XP or Vista.
4. Internet Necessary-An internet connection is necessary and it shud work 'in' Ubuntu. If you don't have such a connection, then it gets really hard to install anything, including codecs and drivers, without which you cannot do many things on it.