Apps: How good is the marketplace? There are 40000+ Apps. More than WebOS (I have a TouchPad). You'll find most of what you want.
Variety: Which phone is seen as the best Windows Phone on the market right now? I would say that depends, thats the beauty of Windows Phone. Samsung Focus S has SAMOLED, very sleak design. HTC Titan is huge (too huge for me) but has a great camera and is well liked, good build quality. The Nokia Lumia has free GPS software thats really good. Lumia 900 is the talked about phone being announced in January.
Homebrew/hacking: I have a little inner nerd.. is any of that possible? Everything is possible. Chevron will unlock the device so you can sideload apps/hacks.
Operating system: How far into the cycle is Mango 7.5 and whens the next version due for release? Mango was released in September. Next version is Tango which is out June or before. Should include more hardware support, LTE, dual core etc... and a video calling solution. All devices should get a pretty seamless upgrade. Mango was very seamless minus the one or two hickups.