I recently asked Jeff Albert what, if anything, was wrong with Travis Hafner's swing. Jeff writes occasional articles for Baseball Analysts and is probably the only contributor to the blogosphere who can speak to a player's mechanics with any real authority. He also is an avid follower of the Indians lineup in general who predicted good things for Peralta and Marte at the end of March.
Here are Jeff's full remarks regarding Hafner, unedited.
This clip compares Travis Hafner's swing from 2006 (left) to 2007 (right). Both swings are HRs to RF off of fastballs.
Although Hafner's OBP is on par with recent year, his SLG% is obviously down. Judging from these swings alone, there does not appear to be anything that is significantly different. Maintaining a high OBP and also a steady K% reinforces how similar the timing of his current swing appears to be to last season.
Normally, I do not like to point out differences unless they are very obvious. This one is getting picky, but it looks like Hafner arrived at contact last year with a spine angle tiltled slighty more towards the catcher. In theory, this would create more loft for fly balls. And the only reason I am pointing this out is because Hafner is hitting more ground balls this year.
Again, going from this video comparison, it is hard to attribute the increased ground balls to a change in swing mechanics, but it is something I would keep an eye on if I was monitoring Hafner's swing on a daily basis.