I would assume that the controls might be geared more for touch than mouse and keyboard for some games, maybe not for something like Plants vs Zombies though. In some cases the desktop version of a game would have more options and perhaps better graphics.
The games for Surface are mostly going to be ports of games from ipad and android, nothing in that library interests me much. They're fun, casual time wasters for when your fiddling with a tablet but not something I need on my PC, metro or otherwise.
Has microsoft embraced the idea of free games & apps that have mini advertisements in them? My nieces have android tablets chock full of games that didn't cost a dime.