As I've written, Ubuntu is tha shiz. But lets be frank - using Linux means you will miss some stuff.
Of course the number one thing is video games. Obviously most computer video games are written for Windows, but there are some work-arounds for that - wine and cedega come to mind.
Unfortunately, right now I don't think Gametap works under either of those solutions, and I really miss that. As a matter of fact I still dual-boot my new PC in order to continue to use the gametap service. I still think that gametap might be brought to Linux. They actually just released the light client for intel Macs - with a promise of the full client coming soon - so I still have hope.
But I don't really miss the majority of the new games coming out. What I do miss are a few select web based services. Netflix has a relatively new feature called Watch Now which allows you to 'gasp' watch content immediately over your computer. The current downsides are that only a fraction of Netflix's DVD content is available, and this feature is only available on Internet Explorer on a Windows XP or Vista machine. I've been working on running a virtual xp installation under qemu in order to take advantage of this feature - more on that in another post. The good news is that Netflix reports that they want to port the service to other platforms. Fingers crossed.
I also wish that there were versions of Orb.com that ran under Linux - no luck so far. Ditto for logmein.com - although I really don't miss that one too much.