Configurability is a huge plus for me. Could you imagine recompiling your phone's kernel? Configuring your phone with dotfiles? Serving webpages and email? Not everyone's cup of tea, and I hope there are more user-friendly options available, but having that power is enticing.
I'm looking forward to better multitasking, and multiple user accounts. Also a more open hardware ecosystem, such as USB and HDMI ports, docks, chargers, USB keyboards, maybe a laptop "shell" I can insert my phone into?
If we look at the "dockable PC" side of things, the developer tools are unparalleled.
And of course free software. CyanogenMod is pretty great already, and Busybox too, but I think a GNU presence will only help. Maybe I drank too much of RMS's koolaid.