I certainly don't hate Vancian casting, but, I wouldn't be sad to see it go. So, here's my list of reasons:
1. Vancian casting gives too much primacy to broad effects and makes specific effects too costly. IOW, if you have the choice between memorizing Fireball or Illusory Script, well, I'll bet dollars to donuts on which choice gets made. Spells with specific effects either get cast after a night's rest or get whacked on scrolls (depending on edition) while the typical spells nearly always get memorized. It makes magic very predictable.
2. It mirrors virtually no genre fiction. There's a reason it's called Vancian. Unfortunately, while I love Jack Vance, I am under no illusions as to how widely read he is. Basing the entire magic system off of a very obscure genre writer from a book that was out of print before I was even born isn't really speaking to anyone other than other big honking genre nerds like me.
3. It's very, very hard to balance. Even after 30 years of development, we still are rejiggering spells and effects to keep them in line with each other. There's just too darn many discrete effects.
4. It's very discouraging to players who don't want to spend umpteen hours reading gaming books trying to figure out what they can do. Even a reasonably low level caster, say 7th level in 3e, could have 20 different spells in his spell book (and this is a pretty low estimate). That's an awful lot of crap to track.
5. Vancian casting makes for hodge-podge casters. This gets reined in a bit with specialist casters, but, by and large, you have core casters with a suite of effects that are all over the place - no theme, no unifying concept. They've got a little of this, and a little of that. It makes casters ... what's the right word? Mushy.
So, there, that's my 5 reasons for strongly disliking Vancian casting.
If I had my druthers, Vancian casters would be replaced by either sorcerer mechanics or the Shadow-caster mechanics from the 3.5 Tome of Magic. (With the mechanics cleaned up a bit.)