Here's all the things I've noticed about spellcasting:

• Gale (Or maybe just Wizards in general) have much more precise animations when casting spells. They don't sway in place like other classes either. Just another way of showing how much control Wizards have over magic.
• Most characters in the game have their own voice lines for casting spells. Even the party members who don't cast a lot of spells by default like Astarion or Lae'zel.
• Bard's spellcasting animations incorporate their instruments. Which means there's different animations for each instrument.
• I haven't tested this myself but I'm pretty sure each party member has their own voice lines for casting Vicious Mockery. Maybe not different insults but at least speaking the insults in their own voice.
• (Act 1 spoilers) During the Green Hag boss fight Auntie Ethel has unique lines for casting Vicious Mockery. And they're different depending on which party member she's targeting.

It doesn't feel like a lot when I lay it out like that but it sure does feel like a lot in the game. The devs didn't need to go this hard just to make magic feel unique.