Google has one of the most basic interfaces on the web. In fact, the simplest pages on Google are their most profitable with most of the revenue coming from search pages and comparatively little arriving from maps, video, Gmail etc
Critical to the success of services like this is firstly the quality of the results - an amazing interface is nothing without solid reliable results to keep users happy. Remember that the search results are a stepping stone rather than a destination.
Secondly it is important to understand that Google has been a lot of people's homepage for some time now and changing how you browse the web isn't an attractive proposition for most people.
I use Google every day and very rarely fail to find what I'm looking for in the first few results, probably because I know how to use Google properly. As more and more people understand how Google works they become more satisfied and less likely to switch.
Any search engine that has a radically different interface is likely to struggle initially but what about if it was backed by somebody like Google? If Google allowed users to 'skin' the search results, integrate Facebook style applications or use a totally different interface then that might open up all sorts of options. They are already part of the way there with Google Coop so perhaps this isn't as far off as we might think...