You know, as I read through the spell list in the 5e PHB, I became solidly impressed by the work WotC have done in balancing casters.
Let me lay it out in terms of wizards:
Buffs:
+At-will cantrips. Ensures that they always have a 'default' action equivalent to plinking with a light crossbow, except that it actually does half-decent damage and scales with level. Also provides a lot of basic utility and flavor-magic, Light, Prestidigitation, &c.
+Schools are now wholly beneficial, although their benefits are by-and-large smaller than they once were, providing a number of thematic effects rather than extra spells.
+Partial spontaneous casting. Prepare a wide range of spells to allow for both general insurance and situation-specific ones, cast a more limited number of them as-needed.
+Spells now often have scaling damage, DCs, &c.
+No more exp costs for spells.
Nerfs:
-CONCENTRATION SPELLS. Holy shit. This one can be a subtle gotcha if you don't read carefully. More than half of the Wizard list is composed of concentration spells, which require the caster to continue concentrating for the entire duration of the spell, or as long as they want to keep it up. In addition to the old ways concentration could be broken, there is also a limit which prevents a wizard from casting more than one concentration spell at a time. If you start a new one, you must stop the old one in the same turn.
Virtually every buff, debuff, and battlefield control spell is concentration. This means that the old dynamic from 3.5e of stacking up buffs before a fight and then layering debuffs and BC is no longer possible. Want to cast Haste on the party fighter? Okay, for as long as you're doing that, there's no Evard's Black Tentacles (of Forced Intrusion), Web, Flesh to Stone, &c. being cast, and vice versa. It also killed the possibility of a caster-tank who buffs themself up massively and then rushes in to hit things.
-Much more limited spells/day, basically no way to increase that apart from leveling up; you really are stuck with one 9th level spell/day.
-No metamagic. It's all a Sorcerer class feature now.
-No Truestrike in the same turn that you cast your rays, though that's somewhat alleviated by it being a cantrip.
-Wish is a much riskier proposition. Sure, there's no exp or material cost, but using it for anything other than replicating an 8th circle or lower spell carries a 33% chance of never being able to cast Wish again.
Washes:
| Greater Teleport is gone. Teleport has the same issues as always, but can be refined to perfect accuracy if the destination has a permanent magic circle that the caster is keyed into, or if the caster possesses material from the destination which has been taken no more than 6 months ago.
| Evocation wizards are actually viable now, in part because the nerfing of BC/buff/debuff spells, in part because of scaling damage and the addition of Int to damage with the Evocation school, and in part because of useful damage cantrips. Sorcerers are probably still better at it, though.
The long and short of it is that the biggest issue with 3.5e casters (overpowering everything else with massive stacks of buffs/debuffs/battlefield control) is impossible now unless you've got half a dozen casters or more as permanent party members. In addition, the general utility and direct-damage aspects of casters have been buffed, such that the loss of their primary role doesn't cripple them, it just makes them more generalist less specialist.
Finally, the large number of spells known and prepared (in the case of wizards) combined with the extremely limited number of spell slots per day and the concentration rules means that decisions are less false difficulty re: resource management (which spells do I prepare? ALL THE SPELLS, because of hyperspecialization in one of the three strongest schools + various items &c.) to real difficulty (Do I cast a real spell now, or save it, knowing that I can contribute in a minor way if I save it? Do I blow my sole 9th level slot to Prismatic Wall this army into the canyon while our allies pepper them with arrows and spells, or save it to Power Word Kill the BBEG once we pound on him a bit? Or to Wish that NPC hero back to life? Do I drop concentration on buffing the Barbarian to hit the newly appeared enemy caster with something nasty, or do I hold off and keep buffing? &c.)