I think that vetoing a trade has a lot to do with who proposed the trade. Granted, that is hard to figure out, but if you do know, here's why: if a player proposes that he give up a star for two decent players, maybe its because he wants those two players to plug holes in his team. However, if someone dupes another player into making a lopsided trade, then that should be vetoed.
For example, just recently, some one (in a private league, and who goes to my high school) proposed that I give him Schmidt for Tejada and Mussina. His argument was that he really wanted Schmidt. I myself do not see why he would do such a thing, but I did not hesitate to accept. But then, probably becuase our league only requires 3 out of 12 player's votes to veto a trade, my trade got vetoed. I argued that if the idiot proposed the trade to me really believed that he was getting a fine deal, then the rest of the league should let him be an idiot. (and who knows, maybe Schmidt will post outrageous numbers, completely blowing away every other FBB player)
My point is that when one player wants a certain set of players, and is willing to give up a better set of players for them, and then proposes a trade doing so, the rest of the league should let that player get what he wants.
PS. I am trying to rework that deal so that it does not get vetoed, and it looks as if Im gonna half to give up somebody else.