One of my favorite features of gmail is that I can preview email attachments without downloading them. This makes it more efficient to determine what I will do with each email attachment. Often times I can reply to an email that references an attachment much faster because I don't have to download to view first. The other benefit is I can also verify I am attaching the right document by its content preview vs just its file name. I simply cannot express how much this feature would be appreciated!
Julian S commented
Even a halfway house would help a lot with this - being able to click an attachement and it download to a temporary folder (or the downloads folder) and open it automatically.