To see what's wrong with UIs, we need look no further than X. Consider the myriad of UIs which run on X, many of which aren't compatible with one another: starting with Athena, through Motif widgets, through competing Gnome and KDE, GNUStep, and so on. There must be dozens of major toolkits historically.

What caused this mess? Openness and the hacker nature did.

A UI needs a bully: someone who says that you will do text fields this way, and if you don't like it, tough. Consistency is absolutely crucial in a successful UI. In Windows, the bully is Microsoft. In OS X the bully is Apple. Open systems by their very nature don't have a single dictator.

A UI also needs a bully to force simpler designs. X is replete with applications that have every single possible configuration option. It's part of the tradition. The problem is that this makes the applications extraordinarily unfriendly to the 99% of the population who want to get work done, in favor of the 1% hackers who like to tweak. It's a rare X application which doesn't suffer from this malady: indeed X does itself. There's no one to tell these application designers that their propeller-hats are screwed on too tight.

I don't know if there is a way to fix this. Simplicity and consistency are the two most important features of a UI, and the open nature of Linux, X, etc. simply run against them. When a bully arrives to the Linux world (such as Google in Android) you start seeing quality again. Without it, you wind up with Maemo.