Testament said:
As has been said, new system
The thing is, so many little things are being changed, things that have been quite constant throughout editions, that this new system is really not even D&D anymore. They may put the name Dungeons and Dragons on it, but it's not recognizable as it.
Example: In 1st, 2nd, 3rd and 3.5 editions, you could talk about how your Gnomish Illusionist ran out of spells for the day so he had to lay low for a while. Come 4e, no Gnome PC's, no Illusionists, and no running out of spells.
With prior edition changes, they might have done little things like take some classes out (Like taking Assassin and Barbarian out for 2e) or putting in new concepts (like Prestige Classes in 3e, restoring the Assassin in the process), but there was a sense of continuity, that what was there was built on what came before. There was respect for the collected body of implied setting information (so called "fluff") and even as mechanics changed some things stayed pretty constant, like damage done from certain iconic spells.
4e takes that respect for the past and trashes it, it builds a whole new fantasy RPG from the ground up and attaches the name "Dungeons and Dragons" to it, in that sense it's a far bigger jump than the 1e to 2e, the 2e to 3e or the 3 to 3.5 jumps, it's an active severing of D&D from it's heritage, it's telling longtime veterans that they are obsolete and WotC wants new players. It's telling a lot of players that what they've come to expect and love about D&D is "wrong" and not fun and was not fun all along.
I've got bookcases and bookcases full of D&D books, I've spent thousands and thousands of dollars on D&D books since the early 90's, and I'm looking at 4e and seeing it as a signal from WotC that my money is no longer desired, they'd rather be selling to pre-teens as a way to compete with WoW