I have to agree, I really like Entourage, but it really is lacking. I mean, if I had nothing to compare it to but Mail.app, I would have to say that it definitely is the best mail app, and perhaps it is for MacOS X. The sad thing is, I think the MS Outlook part of the MS Office 2000 Suite and MS Outlook Express are both superior in functionality since I use both on the Windows platform when at work or school. MS Outlook for Windows and MS Entourage are very similar so the transition isn't so great. But, I always find myself going to do something with Entourage that I commonly do with the Windows Outlook, expecting it to be the same, and have to go oh darn Entourage doesn't have that feature.
One of the majorly annoying differences is the way it handles newsgroups. I love the way that Outlook under Windows does it with the expanding reply tree structure. I just find that method so much more productive. The Entourage way is to just show every message separately with no intelligent way of associating reply messages with one another. Which really sucks when you've become accustom to the other method. Another one would be that the message menu doesn't have an undelete option for e-mail you have unintentionally marked for deletion in an IMAP account. On the up side, I believe Entourage does handle LDAP directory services better than Outlook on Windows.