This is all about my personal use case. I am probably overlooking areas that you care about, and concentrating on those that you don't. This might read as me bashing Ubuntu a bit, but it's really not supposed to. By being honest about these issues, we can hopefully work towards fixing them.

• Proxy support. At Uni/work I am behind a web proxy – the firewall has a default deny policy which means that users are required to go through the web proxy in order to use the web. The problem is that not all applications notice that I have changed the proxy, even when I use the "Apply systemwide" option. I noticed this just now when trying to file a bug report using Apport – the collecting process just hung until I got bored and cancelled it. Launching from a terminal with http_proxy exported worked just fine (actually, automatically exporting http_proxy in terminals if one is set in Gnome would be cool – OSX doesn't do this). I'm not sure where the bug lies here, in applications or somewhere in the Gnome stack, so I don't feel comfortable filing a bug report.
• The speaker output is not muted when I plug my headphones in. I have to go to alsamixer and adjust the headphone volume separately. There's a workaround for this on the Macbook pages of the Ubuntu help wiki but I'm yet to try it. Bug 437150.
• Brightness adjustment doesn't work out of the box. There's apparently a fixed driver on the wiki page that I've just installed. We'll see after the next boot whether it works or not.
• External monitor support is a bit suboptimal. To be fair, I've probably been spoiled by the fact that OSX handles this so well. I have to manually enable/disable the monitor when I remove or add it. OSX detects this automatically and adjusts the windows as required so I can just yank the connector out and go on my merry way. See this Ubuntu QA blog post from Bryce for more on this.
• Battery life leaves a lot to be desired. Sometimes the machine gets really quite hot.

I think that's it. Really these are minor niggles that I can overlook in order to use my OS of choice. I've concentrated on laptop/Macbook specific problems here. We can see that most of the problems I mentioned are hardware/driver specific, and really these are some of the most difficult problems to fix due to hardware manufacturers in the main being difficult to work with, so it's not hard to let issues like this go.

On the plus side, suspend finally works! I can just shut the lid to suspend and open it to resume, just like I can in OSX. This is really great. Also, using my phone as a 3G modem is faster, easier and more solid under Karmic. And of course all applications (barring the proxy issue), work just as they do on any other Ubuntu machine, meaning really well. There's also the benefit that I no longer feel like a hypocrite .

We've a little way to go to overtake the major proprietary players, but with incremental improvements like these, it's just going to be a matter of time.