Enter to open a file in Finder

I'm not sure that's possible or recommended given that it is a basic function of the Finder. Keep in mind that Cmd-Down also opens the file if that is any more intuitive.

Use KeyRemap4MacBook (despite the name, it works on all Macs). In the Change Key section, navigate to For Applications, then Enable at only Finder, then check Return Key Opens a file.

Note: This is a free app that basically does the same as the commercial apps mentioned in Daniel's answer. It has the same limitations regarding pressing Return in other contexts inside Finder.

This behavior is not just intended by Apple, it's forced.

I was trying myself to circumvent it by redefining the shortcut in System Preferences > Keyboard > Keyboard Shortcuts > Application Shortcuts (which is a goldmine, you can use it to assign pretty much anything that exists in a regular or context menu in any program to whatever you want (such as F10 to create a folder in Finder, F5 for refreshing pages, etc. This is unique to OS X AFAIK, and it's bloody awesome!).

As it turns out, you cannot assign the enter key alone to any action. And even if you assign something close, like Option+Enter, Finder will just ignore it. If you look in the drop-down, the Cmd+O shortcut is no longer listed there, and neither is Opt+Enter, it's just blank!

Hacks that used to work to get Finder to do things Apple don't think it's supposed to, are blocked with updates, like this hack to get folders listed before files, which was blocked with Lion: http://forums.macrumors.com/showthread.php?t=1411019

I've previously used PathFinder as I find the native Finder app to be extremely bad, but it's a paid app, and it doesn't integrate very nicely with the system IMO (running side-by-side with the native Finder, destroying the gesture for revealing the desktop so that it's just a blank wallpaper, etc.).

I've just come across XtraFinder ( http://www.trankynam.com/xtrafinder/ ), which is a free plugin for Finder. It's not as feature-filled as PathFinder or Total Finder, but as it's not an app but a plugin, it's much leaner and non-intrusive. It adds tabs, enter-to-open (with your choice of shortcut for renaming, e.g. F2), ordering folders before files, and much more.

It also adds the possibility to refresh the Finder view (which is a more than welcome addition if you've ever tried browsing a network share where a file just appear, yet there's no way to refresh Finder, so it doesn't pick it up until you restart the system (seriously!))

I think it's pretty cool, and it'll sort you out with regards to your problem :)

I am a new Mac user (switched from Windows) and I found Command+↓ to be extremely unintuitive to open a file (Why would it be anything except Enter/Return?).

I Use BetterTouchTool to manage my keyboard shortcuts and set it to trigger the Command+↓ shortcut when I press Enter in Finder. That did it for me (you have to note that by doing this, you cannot use Enter for anything else in Finder, which is fine by me).