K/D can be a good indicator, but dont trust it. Someone with a K/D of 5 can very well be flying mostly lalas, spixteens etc, while one with K/D of say 3 may fly more challenging planes... That does not make the 5 K/D fella a "better cartoon pilot" than the one with 3.
Overall rank can mean that the player is fairly good at, and enjoys all aspects of the game, or it can mean that he is only playing for score. Both are fairly easy to spot (unless they hide in EW/MW all day).
The ONLY good thing about the rank/score thing is that it may prevent atleast alittle bit of the suicide attacks and mindless pork-n-running by atleast some players.