It all depends on a team's needs and how people view a player's value. The mention of Halladay for Sizemore being lopsided is not at all IMO. It all depends on a team's needs and if you match up well. IMO, Sizemore is worth more than Halladay but teams that have SP worth more than Halladay may not need a CF as bad as the team with Halladay.
Then you have to consider the league setup. A Dynasty League/Keeper is run 100% different than a yearly league. For instance in my Dynasty League, one recent trade was Arod/Magglio for Alex Gordon/Justin Upton/Johnny Cuerto. In a yearly league, this is not a trade that makes any sense, in a Dynasty League.........it makes all the sense in the world.
So more factors that a perceived value needs to be considered. I have always believed that when you start vetoing trades, you begin ruining your league b/c then people start to veto everything and the fun factor is taken away.