Shield is mostly annoying because it's easily one of the two best defensive reaction in the game (silvery barbs might be better) - which is not the problem itself, but...
Fighters only get Parry, which kinda sucks. It reduced damage, a smidge, in exchange for a much more limited resource. Parry should add to AC. Shield should reduce damage (or negate magic missiles). That's my feeling on the matter.
The spell that actually annoys me the most is similacrum, not just because it's powerful with no meaningful limitation, but because it really does break the game if you use it to even a good chunk of it's potential. I'm thinking about re-writing it to include incapacitating the copied person, so there are two of anyone running around, just a replacement person whose either a backup (like a discount clone) or an enemy agent. (Like a Skrull, because that's a great hook for a high-level DnD game.)