Baseball Girl at Athletics Nation explains why she doesn't like interleague play:

Yet, I absolutely despise interleague play. Actually, the more I think about it, it might not be interleague play that I dislike so strongly; it might actually be National League baseball, and the A's playing it. Which is why I'd like our team to be left in the American League until forced to play a World Series. I might like interleague play then.

The funny thing about interleague play is that I really feel like I genuinely dislike it for the right reasons. It's easy for a fan of a team that doesn't play well during interleague to complain, but I'm a fan of a team who has a sensational interleague record, and I still hate it! It's also worth noting that our current A's team probably would be better off in the NL right now: Their choices for DH consist of their back-up catcher, Mr. .007 batting average, or a pitcher, anyway, and with the NL's microscopic strike zone, we'd walk our way to victory nightly, but I still can't get behind the idea.

I don't agree with this, however:

I can't stand the mysteriously shifting (and more often than not, small) strike zone, something that seems more prevalent in the NL than the AL. In Nick Swisher's first AB yesterday, there were two questionable pitches taken by Swish, who has a tremendous batting eye, and my feelings were summed up exactly by Swisher's reaction. He turned to the ump, and said something to the effect of, "Seriously?", but using the cuss jar.

Yesterday's strike zone started out the size of Texas against the A's, and about as big as a breadbox for Danny Haren, but slowly shrunk for both sides, until both pitchers needed pinpoint control to ring up a strike. That trend continued into today, where both pitchers were squeezed from the get-go. The Giants reacted by having a walk-a-thon; Loaiza reacted by giving up doubles.

Given there are ML umpires these days, instead of NL and AL umps, I'm not sure why this would happen.