Here's an extract of a conversation between myself and a friend which (hopefully) highlights why good UI design is important to encourage contributions.
[13:03] Roman: argh how do I upload a new version of an image without re-typing all of that text on the upload form? [13:04] Timwi: I don't think you need to retype it all [13:04] Roman: i really expected an "upload new version" link on the image page [13:05] Roman: I can't expect it to preserve the text fields if I don't type anything [13:06] Roman: see, i wanted to make a tiny improvement to an image that's already there, but now I won't do that because it's too hard