Urgh. Are we going to do this every time someone dies? HN already has a tendency to get spammed up with fifty copies of approximately the same article with the same comments when this happens, this is only going to make it worse.
Engelbart was not just "someone". As the other comments in this thread have pointed out, he didn't just invent the mouse. He conceived of hypertext 25 years before the world wide web. He conceived of object-oriented programming. Graphical user interfaces. Watch The Demo ( http://sloan.stanford.edu/mousesite/1968Demo.html )if you want to see a man that was truly ahead of his time.
I think he had a greater influence on the current state of computing than Steve Jobs, Dennis Ritchie, or perhaps anyone in the computing world than anyone except for Alan Turing.
I think it is quite appropriate to morn his death.
Because I like the comfort of familiarity, and hate change or surprises? It's a reasonably common personality trait among programmers (I can put a medical label on it if you want). When I first saw it I spent a while trying to figure out what was wrong with my browser.
Ultimately I don't see why that matters though; understanding why it's bothering me isn't going to make it not bother me.