UI = (you) Vs (i)
Its pretty obvious ain't it? All the times it ends up being what design 'you' the developer thinks of(if at all one bothers to think, as there are many UI examples out there pointing to the contrary) Vs what 'i' the user needs. And when both coincide, you get some amazing products. Example that immediately comes to mind when you say good UI?
Apple of course. iPod, iTouch...do you notice the 'i' there? its no coincidence, its there to show that emphasis is on 'i' the user. (Ok, i just made up that last part, but i think the Apple guys did name it, with that in mind ;) ).
Other good UI examples would be Google(gmail, gtalk, reader), Gnome's SLAB Menu, Flock Browser...et al.
And bad UI examples...well here's a nice comic which gives you an idea. :)
Beauty in simplicity. We know that. But still many of us choose to ignore the KISS principle! Why? Maybe that, all those years of education, robbed us of the ability to think simple. Or as they say, maybe its the case of being afflicted with 'improve beyond use' syndrome.
On a side note: was searching for a Blogger link in Gmail in the quick links section at top left corner in Gmail and found a 'Blogs' in the more section but surprise!surprise! it leads to Blog Search and not blogger! When Photos,Reader,Orkut links take me to my Picasa,Google Reader and Orkut page respectively, isn't it reasonable to expect Blogs to take me to my blogger page?
Wonder when Google will correct this.