i kind of liked the enemy respawns, honestly. it added a frenetic nature to combat since it deterred holing up in a single location to deplete enemies and encouraged always advancing past obvious respawn locations. kept the action level high. they'll have to spiff up the ai to compensate.
no co-op isn't really a surprise to me, i'd have been more surprised if they put it in. the world does need more co-op shooters, though.