Drop-in, Drop-Out Co-Op mode is wonderful. But server advantage, lack of ranked team play (due to co-op mode being favored), 4 on 4, and no difference between playing as the Locusts or Humans makes Gears have a lot of shortcomings compared to Resistance.
Resistance is the first console first person shooter that FLAWLESSLY executes 20 versus 20, ranked team play with squads and parties, differing races to play with, a full selection of game modes, voice chat, and a very addicting ranking system that is far deeper than any achievement list I have seen on XBL (maybe Battlefield 2: MC). That is what makes it special.