Never once in all my years of using iTunes have I wanted to use the mini player. Whenever I tried to zoom the app and the mini player appeared, it would be quickly followed by muttering and grumbling. So you can imagine my joy and bliss at discovering that iTunes 9 finally validated what I had known all along. The zoom button was supposed to act a certain way, and if you want to do something different than the standard, then you ought to use the option/alt key.
I celebrated this change. I called friends I had not spoken with in ages to tell them about it. I wrote a sonnet to the proper use of the zoom button. A party was planned. Ok, maybe not all of that, but I was pleased with the change.
Then iTunes 9.0.1 came out and ruined everything. Yes, I said everything! No, you're the one who's overly emotionally attached to a relatively minor UI issue!
Well, apparently I'm not the only only one. Over on Twitter @zadr and @siracusa reported that you could revert iTunes 9.0.1 to iTunes 9.0's behavior through a command in Terminal.app:
defaults write com.apple.iTunes zoom-to-window -bool YES
This tip was then "retweeted" about 50 times and ended up on MacOSXHints.com which is where I first saw it, moments before breaking out into song and dance.
If you would prefer not to get into Terminal.app, you can get the 9.0 standard zoom behavior in 9.0.1 (and presumably later versions as well) by holding down the alt/option key when clicking the zoom button.