In the same vein as my previous post on Vista, here are my thoughts on Windows 7, which I upgraded my laptop to a while back.
I kept the aero theme on for less than 24 hours this time - the trouble with it is that there isn't enough visual difference between the active and inactive windows. I'm used to a less subtle visual hint about where my typed characters will end up (I could probably get used to it if I had to). Also, the aero window borders are much bigger, which makes them look extra ugly to me. Yes, I know you wanted lots of pixels to show off your pretty glass effect, blah blah. Unfortunately Microsoft gratuitously changed the theme format and didn't see fit to include any way to import XP .theme files, so lots of tedious fiddling with the customization dialog ensued.
I don't like the new taskbar - it quicky annoyed me and I made it as XP-like as possible. Unfortunately Windows doesn't expose a setting to turn off the grouping of similar applications, so until I discovered 7 Taskbar Tweaker, the button I was looking for was never where I expected it to be. The selector thingy which pops up when you hover over a taskbar button annoyed me more than it helped me.
Windows Mail is gone, replaced by Windows Live Mail which is horrible. I found a hack to get Vista's Windows Mail running on Windows 7 which is fine, although: don't use "Take Ownership" to modify the files or you'll break WinSxS and destroy your ability to install service packs (seriously - I had to reinstall the OS to fix this). Instead, rename the "C:\Program Files\Windows Mail" and "C:\Program Files (x86)\Windows Mail" directories and then create copies of them with the original names. You should then be able to replace files in the new copies with no problems (I haven't tried installing a service pack since doing that, but at least it's trivial to undo - just two renames).
Unfortunately, when Windows Live Mail imports from a Windows Mail store, it breaks the Windows Mail store! I had to restore it from a backup, which was extremely annoying.
The new black and white power/wifi/volume icons are really ugly with a classic theme, and are spread too far apart. Unfortunately it appears that users can't change this. I don't like the new way of revealing hidden notification area icons.
The icons in Windows Explorer are much more spread out than those in Vista, so you get to see fewer files in the same space. Fixing this is unsupported and rather fiddly. (Here is my updated ExplorerFrame.dll, although I didn't update it after installing SP1 so it might not work anymore.)
I haven't really found myself using any of the new window management gestures, except accidentally (good job I knew they were there, or I would have been very confused). These might be one of those things that I pick up eventually and then can't live without, but at the moment they are just a vague annoyance. I turned the docking off as doing it accidentally was getting annoying.
UAC is less noisy than Vista's but I still turned it off when a standard command prompt wouldn't let me cd into the "\Users\Andrew" directory from my old Vista installation. I guess Microsoft doesn't expect people to use the keyboard to organize files in this day and age.
You still can't type a path into the dialog that you use to select a directory, you have to painstakingly navigate with the mouse through the hierarchy.
At least the terrible freezing bug doing remote desktop from an XP machine seems to be fixed.
It has crashed a few times which didn't give me a good feeling about it, but I think that was due to overheating (I forgot to reinstall the fan control program that stops this from happening).
It's very fast, which is probably due more to the fact that I upgraded to an SSD at the same time (I didn't want to run an SSD on an OS without TRIM support).