Believe it or not, there are custom ROMS for some of the WP7 phones that allow you to run unsigned code as well as other things - there's a homebrew effort on it that is somewhat akin to the Android scene.

It's not as active, but it does exist.

Unlock, run unsigned and somewhat better applications or modifications - there's a folder mod out there that acts like the N9 PR1.2 method (that was out before PR1.2). Registry hacks, CSS edits, custom tiles, custom behaviors... amongst other things.

Not as much as Android, but a bit more than the N9 scene (which is currently picking up)

Great. Another framework. Let's see if that actually yields more than just a few things like weather apps or note taking apps.