Apple should most certainly integrate sub-folder synchronization as well as displaying the "number of new mail items" badge next to the sub-folder.
Tags = Useless on an iPhone for sorting
Filters/Folders = Anyone who receives any high volume of email uses them. We have to. It would be far too arduous to sort all emails by hand. True, some of them don't get read but I want to know when they're there (such as email receipts for purchases and such). I also want to know when my wife emails me a cute picture of my son (yeah, email – because MMS is nonexistent on the iPhone and AT&T makes you WRITE DOWN some ridiculous ID and PWD). Is that too much to ask? We do everything electronically and we practice good filing habits. I don't think we should be punished for doing so.
Get on it Apple! I've been a loyal customer for many, many years. Get off your high horse and accomadte the needs of your customers. Too much 3G data traffic you say?? You should have thought of that before you inked an exclusive AT&T deal. Get with the program! Even though we didn't pay retail, we spent too much money on these "subsidized" iPhones to be crippled by such nonsense.