I'm pretty new to duck ownership, but I raised chickens throughout my childhood. Ducks and chickens are WAY different. For starters, I think ducks are smarter and more personable (sorry chicken lovers!) Ducks are very silly, silly creatures! They'll keep you entertained for hours. They can also be very affectionate and can be trained to some extent.
They do have different needs, though. First, obviously, is dietary. Second is access to water, both for bathing, drinking and bill-flushing, and third is company. Ducks do best kept in groups, chickens tend to be better adjusted if they're flying solo...er, no pun intended.
Again, take my words with a grain of salt, I'm VERY new to duck-dom but I'm relaying my own experience as well as all the research and book reading I've done!