(I'm not saying it has to be Enter, but a two-key shortcut seems very bad for an operation that is used so often, especially considering you can't even press them with left hand!)
Same goes to Delete.
Note that you can also use Cmd-down_arrow, because Cmd-up_arrow and Cmd-down_arrow can be used to navigate up and down the folder hierarchy, and the latter also works if you have a file selected rather than a folder.
Edit: Cmd-Delete will delete a file. I think the philosophy is that Cmd is used for commands, and Delete by itself is meant for text editing. One thing I like is that Cmd-Shift-Delete will empty the trash. I'm unaware of a keyboard shortcut for emptying the Recycle Bin on Windows, or any way to avoid the confirmation dialog when doing so.
Wait, how do you "navigate down the folder hierarchy" if you don't select a folder? There is no deeper hierarchy.
BTW, I didn't even know about Cmd-Enter until aidos mentioned it; so that makes 3 different shortcuts to open a file. Convenient! :-)
Windows is similar but with alt: Alt+up=up, Enter=down (or open). And there are also alt+left and alt+right as back and forward.
Hitting “enter” to open a file makes it too easy to do it accidentally. What if I didn’t mean to start up a huge app like a pro video editor or something, which takes five-ten seconds to start up?
Cmd-down-arrow is pretty darn easy to invoke from the keyboard, but not so easy that you risk opening a file you didn’t mean to.
If you have to start your post this way, maybe reconsider if you're going to add anything.