Why isn't there a Search button? It's actually disconcerting to simply hit the Enter key after I type in my search terms. It's like, "I'm hoping this will search, but I'm really not sure."
If you enter some dummy search in the box, you'll be presented with the search results page. There you will find a search button at your disposal.
I don't feel that this is much of an issue really. It is a UI/UX design decision that was made because pressing enter after entering a search query actually is intuitive IMO. I have a feeling that people would agree with me on that.