Finder: I wish I could set certain folders on certain view mode. For example, I prefer most folders to have columns view, but my Documents and Pictures folders would be better using cover flow. Nope, I have to switch manually depending on where I'm at. Annoying.
Preview: This is probably one of the most useful apps in the OS. Why? You can crop and resize images with it. You no longer have to run iPhoto or download other software to do simple things like cropping and resizing. There is also a feature called instant alpha. I haven't really played with it, by the idea is to be used for getting rid of backgrounds, something that was usually only doable in advance photo editors.