I'll give you a few examples. I was an iPhone user for 2.5 years and switched to an Android phone a month and a half ago. (thank you Google I/O!)

First, there are 2 calendar apps and 2 email apps. One calendar app is just called "calendar" and the other is called "corporate calendar" I think the idea is to have the exchange calendar in one app and the regular one in the other. But this is unnecessarily complicated. Why not just one calendar app like the iPhone which has all calendar events? Likewise, there is an Email app for any non-Google accounts, then GMail. So when I get an email, it may open in either app which have a completely different look and feel. While that's mostly fine for a geek like myself, it is confusing to any normal users and is a UI mess.

And there are many other things throughout the interface which lack polish. The phone app, for example. When in a call, my fiancee accidentally hit the home button. She then went into the phone app to get back to the call because she had to enter things on the dialpad to continue. (she was on the phone with a 1-800 number's automated system) So she went back to the phone app to get back to the call. But that opened to the call log. So she went to the dialpad and tried entering the numbers like she was told, but that dialpad is the dialpad for initiating a new call, not for the current one, so it didn't work. The system timed out and she had to make a new call again. For reference, the way you're supposed to do it is to pull down the notification bar where to get back to the current call.

In contrast, on the iPhone when you hit the home button, there is a green strip across the top of the screen that says "Return to current call" so it is trivially easy, and if you go back to the phone app in the middle of the call, it takes you right back to the current call.

While on the phone app, when an incoming call comes in on it is difficult to figure out how to merge calls or hang one up and answer the other. To be honest, I still haven't figured it out yet. On the iPhone, the call comes and it has 3 big buttons: Hold current call and answer, Merge calls, End current call and answer. Trivially simple, good UI.

Other things, the Android virtual keyboard SUCKS compared to the iPhone. It's predictive text is just not as good and my typing speed has been cut in half with this device because it fails so often.

The gallery app, while flashy and 3D is slow and laggy, and I yearn for the simplicity of the iPhone gallery. I don't need my pictures flying around in 3D if it detracts from usability with performance issues.

Often on my Droid, I hit the home button and all I see is my background. I often have to wait a few seconds for my shortcut icons to load, then a few more for my widgets to load and refresh. It's terribly frustrating. And sometimes they don't all load, I just have to swipe on the screen to get it to work. I shouldn't have to wait for my home screen to load.

Mobile Safari is just a much nicer browsing experience than the Android browser. I've even tried Dolphin, but find it leaves a bit to be desired. It is just not as good at zooming, panning, etc. as Mobile Safari which is HUGE for a browser on a smartphone. With so little screen real estate, I don't want to spend my time frustrated with that aspect of the browser.

Overall, the OS just feels sluggish in nearly every way. I don't know how to quantify it, but everything is less smooth and responsive. It's sad that my 2 year old iPhone 3G feels faster and more responsive most of the time than my new Droid. I even tried overclocking the thing to 1GHz and it still felt like a dog compared to the iPhone. And it is almost certainly all software because the processor in the newer Droid is certainly faster than the iPhone 3G, especially when overclocked. But it still felt slower.

Overall, there are many more little UI problems all throughout Android which make it far more frustrating to use. I don't hate it entirely, I wouldn't have switched if I did, but it is annoying and I am willing to live with it because of a) Verizon and b) the Hackability. But I'm a big geek, I simply would not recommend an Android phone to most of my family/friends and that is a big problem.