Weird bug: Cmd+Tab sticks in Mavericks
The command ⌘+tab ⇥ app switcher lives in Dock.app, as many other things do, like the desktop background, menu bar, Spaces, Exposé, and most obviously the Dock. So to fix this the easiest way would be to simply clear the Dock preferences using this Terminal (/Applications/Utilities/Terminal.app) command:

defaults delete com.apple.dock; killall Dock


For anyone not familiar with the defaults command, it is used to set or remove preferences, most of the time hidden ones. This line simply uses the defaults command to delete the preferences associated with the bundle ID of com.apple.dock, which is just a unique identifier for Dock.app. If you want to be a little more technical and see if you can salvage your Dock's app order and Stacks, try running

defaults read com.apple.dock


and looking for anything suspicious or corrupted, and then use defaults delete on the offending key.