If recent trends are to be believed, Ubuntu's in decline. And Unity's pinned as the main culprit for this. Unity or no, it looks like the decline started before Unity even existed.
Personally, I don't mind Unity (too much). Gnome Shell looks good too. I'm currently using Unity but I've tried both and each has its quirks to work with. With Gnome 3 officially no longer using the classic Gnome Panels, you can hardly blame Unity for all the troubles.
It's probably something more fundamental.
For any OS to gain support a standard API is needed to attract developers. Just look to Windows as an example. I don't use Macs but I bet they have a standard API, too. What do we have?
No half-baked features. Let's face it: Unity is rather half-baked at this point. It's got annoying bugs here and there that can really put people off. It's not enough to make me jump ship yet, but it's still annoying. For something as big as the desktop shell, being half-baked is suicide.
It seems we're changing our default application set. Again. After moving from Rhythmbox to Banshee in the move from 11.04 to 11.10, we're apparently moving back to Rhythmbox. A little consistency in the default apps goes a long way to inspiring confidence, I think.
Balancing the oft conflicting interests of the Ubuntu userbase is a tough job, but the developers have done it before. Maybe a step back to reassess the situation is needed.
Hopefully things will get better again with Precise Pangolin.