I do not believe the keyboard can be changed and theming is really only done by changing the colours and choosing what cool tiles you want on your home screen. It may sound somewhat boring but once you add social networking accounts and pictures, the phone really becomes yours. I personnally like how the tiles are constantly in motion and how slick the OS is. The built in features like Bing Vision (I don't really care for the actual search aspect of Bing), the People Hub, Office, and the integration with MS services in general are a huge plus for me. Despite what some will tell you, the WP ecosystem is very strong (being part of the MS ecosystem) with services like Skydrive/Windows Live, Zune, and Xbox Live. Since you're comming from Android, the smooth, reliable, unified OS should be a joy. I haven't had my phone crash in over a year (back before the NoDo update when WP was very young).
As with any OS there are some negative aspects. The lack of customization doesn't really bother me but can be a deal breaker for some. The current hardware is fairly restrained so you won't find features like HDMI out, HD screens or 1080p video recording. Some apps that exist on other platforms cannot be found on Windows Phone but there are usually alternative apps and the marketplace is growing. Some people desire a file system on their phone (I've never understood why anyone would need to manage files on a phone) and there's no mass storage options either. Without becoming a developer with MS I believe there is no way to sideload apps anymore either.
These are the best pros and cons I can think of at the moment and overall I've found Windows Phone to be the best mobile OS out there. My little HTC Surround regularily holds its own, and often beats, new phones like the iPhone 4s at day to day tasks.
Hope this helps!