It gets worse on something like Windows 8's Metro where you get to see like 20 different square blocks, each with a different color, that may update all the time too. Try not to get a brain seizure from that.
But as always, your brain maps that information jungle to something familiar if you keep using it and it's not that complicated anymore.
After giving it some time, it's no different than pinpointing maps icon in home screen where there are other icons each with different color. At least I haven't heard anyone using Metro interface enough complaining about finding things. There are just more info on a given Metro home screen than iPhone or Android home screen, and I'm sure iPhone screen is quite complicated for someone using a Nokia 3110 for sometime.
Perhaps for developers it may appear out of place as we are use to complexity. But I believe for a regular user this is killer, a simple spaced out flat design helps make the decision making faster. Again it depends upon how well you use flat ui theme. In flat UI I believe minimalism and spacing is very important.
I recently started using windows phone & win 8 pc. After using it for sometime I have realised that if you are into only few apps & want to use it effectively & productively then this is a great UI. These UI do not encourage more apps usage.
So next time if you are building a simple tool, flat design might be really cool. It makes users believe that there is less effort & it is faster. Again you need to master it to take good advantage of it like very other style. Signup page using flat ui is also a great idea if you ask me.
OT I still find oversized icons, as in Gnome 3, Unity and Windows 8, almost impossible to grok. I never used to be a fan at all of deep nested menus, but a menu is so much more succinct and easier to grasp.
It might just be something about the visual cohesiveness of all the elements.
I have played with Win 8 Metro only a bit, so I can't speak for it's practical usage, but I own a WinPhone8 (Lumia 920) and the UI i think is not at all overdone, and other than tiles having their non-standard images sometimes, it's all very consistent (except for the few apps that are e.g. ports from Android or iOS, which use non-standard UI, but that's the developers' faults, not Metro's).
