Mail handling can be unstable sometimes. You may see emails show as "No Sender" or "No Subject". Quit the Mail app and restart to fix.
Mail previews are not as accurate as 3.x. Some HTML content doesn't load. Scrolling and zooming sometimes fail. This is at the top of my list.
Launching the Mail app from a mail preview will crash the Mail app.
The "Show While Playing" setting is getting ignored.
Twitter plugin replies or new tweets don't work right now if you are using a passcode lock. It will trigger the passcode screen to show.