Right now, martial classes can improve weapon skills when they observe an enemy attacking them. I think it makes sense for this to be expanded to all classes and abilites. And not just when they're used against you, but when anyone around you uses them at all. So you see your groupmate block with a shield while you're ranking and get a little insight. Or an enemy dark elf uses their inherent faerie fire on you and you figure something out about your spell version. This would probably not end being a large number of your total skill ups across a character's life, but it seems like it would make the world feel a little more lively and connected.

It's already easier than ever before to pick up on skills.

And we don't think "seeing" to learn works with all skills, concepts, spells, etc. Personally I'd keep it to martial things since it fits the idea.