This is a big of a nag, but mostly truth. Hear me out.
Most of users' problem with the email is the lack of spiffy graphics. I don't understand why people rant and rave onver the email because it doesn't have spiffy graphics. Also, this extends to HTML formatting. They want to be able to use god awful formatting and colors and fonts that make your stomach churn, or at least hurt your eyes.
Now that I did my mostly truthful rant, let's get to business:
I have also heard about problems configuring their email client for certain features. Guess what? Forums require the exact same thing! If you want people to reply via email to a different address, you normally setup a reply-to field in your email client. This is usually done for better notification of replies. With forums, you register your email that you recieve notices with, then you have to set for every topic oyu start (or reply to) that you want email notification. This is expodentally more complex than email!
While admin-set global stickies are something that only forums can do, that's basically the only thing going for them (and we could just instruct users to setup a sticky filter based on a string that only admins can put into subjects, save replies).
User-set custom stickys - These are a pain to set with forums, that is if the forum software supports it, and 99% of forums software doesn't. Users like this, and switching to a forum can really piss them off with this. With an email client, you have more flexibility, cause depending on your client, you can setup these by user, subject, regex on the user or subject, if a attachment exists or not, body of message, size, time, date, who is in the 'cc' list (to hide those not sent directly to the list), and so on. The only forum software that I saw that did this would only do it if you selected a thread for a personal sticky -- this isn't really flexible compared to email.