I realize this is an old thread, but I just got a brand new iPad 2 this week and share the same complaint as the original poster, though possibly for different reasons. Here's why I don't like the automatic preview display:
1) If I'm currently using my 3G connection, those email previews that I'd just assume not see keep using up the bandwidth on my data plan. So it is potentially costing me money (I don't know yet how much bandwidth I'll use in a month).
2) If I'm sitting next to other people, I don't want an email that might contain inappropriate OR sensitive info to automatically display the message content just because it happens to be the next message in the list after the one I just deleted. This is potentially a serious issue if I'm sitting next to a young kid and I open an email that a friend sent that has an inappropriate picture in it or if I open an email with sensitive proprietary information and happen to be sitting next to a competitor. Either of these situations could cost me a LOT more than just money. I'm sure someone will tell me that I shouldn't look at my email at all in such situations or setup filters to keep that data off my iPad, but we all should know that filters don't always work 100% and part of the point of getting a device like an iPad is to be able to use it anywhere.
3) As someone else said, the preview pane takes up enough width that you can't always see enough of the subject line to know what the email is about for the messages in your list.
This is a lot different than having a preference for a button to be blue instead of green. I've only had it for a few days so far and I like the iPad quite a bit so far, but this preview pane thing is a pretty big negative to me. And as someone else said, using the gmail web interface isn't an alternative because they made it look like Apple's Mail app on the iPad and the desktop version you can switch to currently doesn't work because the scrollbars aren't present leaving you no way to get to any emails that require them. Gmail shows pages of 50 messages at a time (by default), while the iPad will only fit about 25 on the screen at a time - next page jumps to email #51, so there's no way to get to email 26-50 and the equivalent emails on subsequent pages. I realize I can adjust gmail to display 25 messages per page, though I'm not sure if 25 actually fit anyway.
