Very good points in the following article: Why Google Will Never Be Good At Enterprise Search | SocialComputingMagazine.com. I like to think of the difference as: Publisher-centric search with influence given to the reader/subscriber for internet search. Data and hierarchy centered search for enterprise search.
As a long-time desktop applications developer, I have a lot of experience shrugging my shoulders at the “Enterprise” side of development. However, the reality is that the “captive audience” mentality dooms you to mediocrity: Wide Awake Developers: Why Do Enterprise Applications Suck?.