I run three leagues through Yahoo and find the trade veto policy to be a very hot topic at times.
My stance on it is very liberal. I let almost all trades go through, but there's an exception to that. In all my leagues I'm almost 100% certain I do not have any managers doubling up with two teams in the league. Given that knowledge the likelihood of collusion is very low. That doesn't mean that someone wouldn't get disinterested and try and dump their squad. I have a zero-tolerance policy for team-dumping and/or collusion. If anyone is found to have done this they're immediately locked out from the league and banned from further participation from that league or any other league of mine (NFL, NBA). In the case of team dumping I don't just punish the team that dumped but the team that took the deal as well, for they are just as guilty for taking a trade that would undermine the integrity of the league.
I'm under the firm belief that managers in my league are making intelligent, well-thought out deals to better their position in the standings whether it be short or long term. I feel if two managers are able to reach an agreement on players, that they should be able to make the deal, regardless if its a little lop-sided. This isn't to say I don't get my fair share of complaints every now and then, but it would take a large majority of managers from the league(at least 50%) to get me to veto a trade. I find that most trades that get bitched about often aren't that big of a deal after a few months. Seriously, have you ever looked back at trades made in April and May once the season has ended? A lot of deals that looked even steven suddenly look terrible, while a highly contested deal looks about as fair as can be. I know hindsight is 20-20 but I think people tend to take the narrow view on judging trades sometimes.
Also on the issue of the commish pushing through trades faster than others, I alleviate that problem by simply letting every trade run through the entire trade approval period. Once its gone through the entire period Yahoo will push it through for me. This way it doesn't show any signs of favoritism.
I see wrveres posted before me. He just recently joined my NL-Only league. Well I guess you know my stance on trades now!