To me a bad port can have one of the following:
- Game is really buggy or doesn't work.
- The graphics/sound/etc. are inferior to the other platform for no real reason. It can have the same quality, but it can't have worse
- Controls obviously not tweaked for keyboard + mouse. Some games I can't fault for this one however, like Assassin's Creed or Mirror's Edge.
- Limited system options.
- Forgetting to change the UI to reflect that I'm not using a controller (though this is more laziness)
The game also has to have existed on another platform first, before moving to PCs. If there was co-development, it's not a bad port. It's just bad.