I didnt realize how important Shane was to the show until I watched it a second time through. Constant tension is a GREAT description of what he bought to the show. You could cut the tension he had with every character with a knife and it blanketed every scene he was in.
and on the other hand I for one was Glad they killed him off when they did.. His "shtick" had grown old.

Looking back, most characters that been taken out had worn thin and, IMO, needed to go.. Lori, Shane, Andrea, Dale, the Guv...
Herschel was a little disappointing, but needed for his girls to grow up..
The only one I can think off that was taken out that I felt should have stayed is Merle.