So, let's look at 2e vs 3e D&D.

Similarities

2e and 3e both have Vancian casting as their primary magic system. Both have mana based alternatives for Vancian casting. Both have vast libraries of published spells, and allow casters to research their own spells. Both have Wizards which have the option to lose access to whole swaths of spells by "specializing" in one of the 8 basic "schools" of magic, and Clerics with the capability to have mildly differentiated spell access depending upon their faith. By default, clerics in both systems have automatic full access to all their spells. Both have alternate "magic" systems that don't use Vancian magic at all (ie, psionics).

Differences

2e also has various Wizard specialties for things like each of the 4 classic elements, thought, wild magic, etc. 2e has multiple mana systems to 3e's one. 3e has more alternate magic classes, and more unique, distinct systems for those classes. 3e gives Wizards limited automatic spell acquisition, plus the option to purchase additional spells in shops, while 2e Wizards were mostly limited to random spell access from loot, and spell research of unique custom spells. 2e Wizards were further limited by "maximum spells known" and their chance to fail to learn spells. 2e Wizards had their "opposition schools" (which school(s) they lost access to when specializing) predetermined; 3e specialists get to choose their opposition schools.

Builds

2e had a lot more variety in builds. This should be rather obvious, given that what spells you had access to was largely random. It was much more like building a MtG deck in a Limited environment (like sealed), but then some people wrote their own cards to augment their deck.

With mandated opposition schools, specialists played very differently from one another (as opposed to 3e specialists, who all ban Evocation).

In 3e, most of the most broken options are right in core. Thus, in high-op play, "how many books you have read" has limited impact on your final caster. As this was not the case in 2e, there was more variety even in high-op in 2e than in 3e.

Both have a large variety of possible builds, and a large variety of viable builds, with 3e narrowing much faster on the "likely" and "high-op" builds. 2e, however, arguably had the drawback of seeing a few more "not viable" builds in play than 3e.

In play

Eh, I'll come back for this later.