The PC will support mods:
Bethesda's fifth Elder Scrolls game, like Oblivion before it, will
allow for extensive player customization, according to the developer.
"We've always been impressed with what the community has done with our
tools," a rep for the company said on its forums, revealing, "Like the
Elder Scrolls Construction Set for Morrowind and Oblivion, we plan to
release [a] Creation Kit so you guys can mod Skyrim."
The immediate predecessor, Oblivion, has seen countless mods --
everything from new characters to online play. We can only imagine
what the Skyrim community will manage to create while we wait for
Bethesda to finish the most ambitious Skyrim mod, Fallout 4.
The PC also supports console commands which can be convenient if there is a bug with quest items.
The Xbox 360 has the first 2 DLCs exclusively for 30 days.
As far as I've found all three versions support achievements/trophies.