As some have said, the problem isn't that there is change, Realms fans do expect the timeline to march on and for the world to have some changes. . .
The problem is when the changes are so big that the infringe on what makes the Forgotten Realms feel like the Realms.
Killing off Mystra, destroying the weave, completely reordering the planes/multiverse, driving most wizards insane, Helm being slain and causing major shifts in alliances/loyalties amongst the pantheon in response. . .and it being implied these are long-term changes for at least the next century, that's too much.
It seems extremely un-Faerunian to permanently kill off Mystra. She has died twice before in the timeline, once as Mystril when killed by Karsus, the second time when slain in the Time of Troubles. Both times she reincarnated, the second time her mantle of divinity was entrusted to a mortal who was elevated to divine status. It was made clear that the omnipotent Overdeity of the Realms sees Mystra as a key part of the Realms, and while the current Mystra can die, Mystra in some form will continue. This whole "spellplague" throws out continuity and what was already established that an omnipotent overpower watches over the deities and lets them have their little squabbles but steps in when their wars threaten to bring cataclysm to the world.
Most of the changes in the 10 year jump in the timeline aren't that bad, things that could be expected, some wars, some alliances, a few minor deities dying in wars amongst themselves, it's the last two years in that timeline, especially the very last one, that has fans hackles raised.
It seems like WotC is going out of their way to smash the Forgotten Realms with a sledgehammer and put the remaining pieces together to fit 4e design philosophies such as "points of light". As Kae'Yoss said, the things that made Forgotten Realms distinct from other settings such as Greyhawk get lost in such a shuffle.