Here, I'll throw everything up front. Virtually every complaint I've ever seen about the ranger falls into these three categories:
1) The class is underpowered/out of balance compared to the other classes. ('The ranger is shafted!')
2) The abilities the ranger is given don't match a given concept of the wilderness warrior. ('I don't see rangers as two-weapon fighters/spellcasters/whatever.')
3) The ranger is 'front-loaded'; the most powerful abilities (the Track, Ambidex, and TWF feats) are granted at first level, and nothing of consequence comes in after that (at least not for a long time). Result: a proliferation of characters with one level in Ranger and the rest of their levels in Fighter, Rogue, and/or Barbarian. ('I'm tired of seeing mucnhkins take one level of ranger for the TWF ability!')
IME, I think 2) and 3) are pretty good points. Some added flexibility in the class would be a big improvement, and moving some of the special abilities to 2nd and/or 3rd level would make the class more interesting to develop (as well as less attractive to min/max multiclassing). IMC, I added some minimal tweaks to the class to do this (virtual feats don't apply until 2nd level, and there are choices other than Ambi/TWF available).
I heartily disagree with 1). Well, mostly, anyway.

The ranger may be just a bit underpowered ... but not enough for me to worry about. I think that the root of this complaint actually lies in complaint #2. Players have a preconceived notion of what the ranger should be, and play the character according to that preconceived notion; then they complain that the character isn't good at filling that role. Playing a ranger character along the lines of how the class is actually designed makes for a pretty decent and effective member of a party.