The changes to the vanilla game that bother me to the point of not buying Dawnguard for the PS3 are the loss of hostility to stage 4 vampires and the new random event. Possible non-quest spoiler discussion follows.
So if I ever make another vampire character I will do it without Dawnguard. I actually have a vampire who lives in Solstheim. I’ve never written a word about her because she’s
so damn boring. There is no conflict in her life. I’m pretty good at filling in the gaps with imagination, but a blood-starved vampire who gets a friendly greeting from the guards and isn’t much bothered by the sun? I liked it better when they had to feed or deal with the consequences.
Yikes, that was kind of a rant.

I should mention that Dawnguard also has my favorite armor and weapons by far as well as a lot of very cool characters, events, locations, and stuff. And I think that if you try it and decide later to delete it, you can still play old characters whom you did not load while Dawnguard was installed? Is that how it works?