One of my very first posts here was about arbitrary class restrictions in the history of D&D. I later discovered that a bunch of my claims were pretty inaccurate. When I wrote it, I must confess, I was pretty rusty on my knowledge of the TSR-era editions, and rudimentary in my knowledge of the WotC-era evolution. These days I am, to some extent, catching up with the game, under the excuse of getting to know the new 5e.
I have never had personally to do with edition 3.X, and I have barely peaked into Pathfinder in the last year or so, so to me the choice between those and 5e is not particularly dramatic. I have read the Basic Rules PDF for 5e, and browsed the physical books. All in all, I like the way the flood of class options from those earlier editions has been streamlined, and the main innovation in terms of mechanics (the advantage-disadvantage thing) seems reasonable enough, in spite of being (as far as I know!) entirely unprecedented in D&D. I had fun creating a bunch of characters, even though I found out that when it comes to arms and armour, 5e is as arbitrary as ever, only, this time it is a matter of race rather than class: an elf wizard can use swords and bows without having to multiclass, a human wizard cannot. Funny, but not really a deal breaker for me.
The fact that the magic system moved away from Vancian casting does not bother me in itself. In fact, disconnecting spell slots from spell memorization basically changes slots into something like Power Points (see below), a repository of daily magic fuel that you can spend as you see fit. An increase of flexibility, and it is also easier to rationalize in setting.
One aspect of the magic system, though, strikes me as entirely undesirable. Can’t you guess? At-will cantrips. The very idea of magic use being free from any resource constraint is, I think, hazardous and hardly ever adopted in RPGs. D&D had Vancian casting, Rolemaster and Basic Role-Playing had Power Points, GURPS had fatigue. Even the powerful and free-form Magick of WW’s Mage series was in fact limited post factum by Paradox Points and other mechanics (basically, if you used Magick too nonchalantly, bad things would happen). It is not merely a matter of game balance among classes/character types. It is also a matter of the role of magic users in the world and in gaming sessions: if any spell of the slightest utility could be used all the time without repercussions, you must expect a PC to use it repeatedly in every single session, perhaps multiple times in a given scene. Many supporters of 5e on the Internet frankly admit that such is the case with cantrips.
If we were only talking utility spells, the thing would not be terribly serious. Yes, a spell like Mage Hand, if usable at will, can in fact trivialize a good number of classic lower level D&D situations: think taking keys from a guard, or pushing a lever without touching it. But things get worse when you look at the great number of attack cantrips, some of them dealing an incredible 1d10 or even 1d12 damage! Defenders say “Man, it is still less than a good crossbow…”. Of course it is, but it is still a pretty decent approximation! This means that first level wizards are now going to be employed in any given combat as (second rate) archers/crossbowmen and to shoot stuff from their hands all the time.
I can see two problems with this fact. First, mechanically, it radically changes the most fundamental reason for having classes in D&D: not every character type should be capable at doing the same as their companions in any given situation. Wizards are not supposed to engage in combat and kick asses every single time: they already can do many things that no other class can do, so empowering them further in combat is unfair. Second, a narrative problem, if you like. A wizard who shoots acid/ice/fire from his hands at will, and (as a consequence!) does it dozens of times each session, does not resemble any fantasy character I can think of. It resembles, if anything, a pretty boring superhero. So, I know that kids these days call you names when you say what I am about to say, but I will say it anyway: this is not D&D any longer.
Nota Bene: I am aware that at will “0 level” spells are present in Pathfinder, and I would extend my perplexity to that game as well, if it was not for the non-secondary detail that those spells are fewer and inflict negligible damage. I can then see them used in play as mere last resort measures, rather than the default actions every single round. So, if anything, Pathfinder appeals me more than 5e under this respect.