Without good results, it doesn't really matter about the bells and whistles. I use a search engine to find information, so it better do that extremely well. For example, I just couldn't stand using DuckDuckGo (aka Bing) because of this, and went back to Google. Bing consistently failed to find information the information I wanted, while Google had it on the first page.
So, after your engine returns as good or, ideally, better results than Google, you can start thinking about other features.
One feature I'd really like is to be able to tweak my result set. Something like if I search for "AC DC", I get a bunch of results about the band "AC/DC". That's not really a bad result given the input, but in this case I was after an explanation for the electrical terms.
So I'm thinking some ability to mark one or more of the results I don't want and say "not pages like this", and it would cull those talking about the band, in a weighted manner. Or some other way to help me find the information I want when I search for some ambiguous terms.