This has been a fun discussion to follow, with some good points made on both sides of the argument. Personally, I've always viewed balrogs as having wings (which may have been influenced by my seeing the Bakshi film before reading LotR), but probably wouldn't have any problems accepting a wingless version in another translation. I won't go into my other reasons for being a winger as MM has pretty much stated them (and more) already.

I'm not sure how well the "balrog must be man sized to stand on Durin's Bridge" theory could hold up under close scrutiny. I'm a pretty big guy, and while not the most agile person on earth, I know for a fact that I can stride across and maneuver confidently upon a 2 x 8 wood plank (I build decks as a side job) without fear of losing my balance. If the balrog to the bridge were to be roughly the same proportion as I to a plank, that would make the foul beastie considerably larger than Gandalf. Sure hope I haven't confused anyone with that line of reasoning.

As for the "how could a balrog 'fall' all the way down to the depths of Moria if it had wings" line of reasoning, it could be answered by viewing fighting birds. I've watched birds lock into combat while flying, and their seeming lack of concern for flight was apparent in the way they plummeted straight toward the ground (before breaking apart in time to avoid impact). I would imagine that if the balrog and Gandalf were scrapping during their descent, the situation could be similar. But that's just one way to look at it. There are too many variables left out of the narrative for anybody to prove much of anything. IMO, that lends a nice touch to the mystique of Middle-earth's immortal denizens.