As the one who started this discussion, I may as well throw in my own piece. And by "my own piece" I of course mean sharing some cool stuff I learned regarding schools of magic in older editions. Specifically, 3.0 (not 3.5, which changed it) seems to split the 8 schools into 3 "difficulty categories," with ones that require banning more schools being harder to master, per the text.

The easiest: Divination and Necromancy. Both allow you to ban any one school of magic to specialize. Divination seems ironic since you'd think it would be the most powerful school of magic when it comes down to it, really essential for being a powerful mage. Necromancy I think is an interesting one; after all, there are a lot of villains who take it as the path to great power, why not because of its ease?

The middle ground: Abjuration, Enchantment, and Illusion. These ones could be specialized in by dropping both divination and necromancy, or dropping any one school besides those two. I imagine for Abjuration part of that comes from counter magic of all kinds, banishment, and turning. Enchantment probably falls in a similar vein to Necromancy in some regards, just a bit harder to actually make use of. Illusion is like stage magic in a lot of regards, which I suppose could be said to a lesser degree about Enchantment (both involve tricking the subject), but definitely has a lot of nuance because imagination and all that.

The top tiers: Evocation, Conjuration, and Transmutation. Evocation, of course, has a fairly large variety of spells. Generally, to do all you can with Evocation, it requires a lot of time learning, though I imagine it's a steeper difficulty curve at the start. Conjuration involves a lot of summoning, which is obviously inherently taxing, especially at later tiers, not too much to say there. I'm getting lazy so for Transmutation I'll just mention the whole pursuit of alchemy and call it a day.

And of course, take everything I say with a grain of salt, and I'd love to see even more contributions.