One of the most important features I would miss is "Fetch Mail". I'm managing several domains and currently I'm fetching all of my mail to one mailbox after I log in. In the configuration I specify which folder to put the fetched mail for the particular account.
Another important feature is when replying to list email I have an option to reply to the list as well as the ones already supplied. When I Reply-All to list mail the client doesn't obey the Reply-To rule and will add every address to the list of recepients, including the list. This has the benefit of allowing you to reply to an individual without hassle if the list is set to munge the Reply-To.
-- The dashes should be optional.
Earnie
P.S.: This one is sent with RoundCube Webmail client.