I haven't gotten to play 5E yet, but I'm really looking forward to it. 3.5 is really good, but it has some major flaws. It's very rules-heavy, classes are unbalanced relative to each other, characters must have certain magic items to be playable, the goal is always to figure out how to get one more bonus rather than any interesting abilities or items, and combat is rigid -- if you're not standing still and making a full attack, you're probably doing the wrong thing.
While Pathfinder makes some major improvements, it makes the class balance issue even worse, giving nice new things to all classes, but the best nice new things to spellcasters.
4E makes balance its main focus, to the detriment of gameplay. Every class has the same set of powers, with each class having slightly more of one type of attack or effect.
I think 5E, while not perfect, is the best version of D&D yet. A 20th level fighter is a very playable class, while still feeling like a fighter. Classes aren't perfectly balanced (fighter and spellcasters are better than monk, barbarian and rogue, which are far better than the poor, left-out ranger), but the differences aren't nearly as great. Levels are no longer exponential. In 3.5, a dozen 10th level wizards would have no chance against a 20th level wizard. That's not true anymore. Magic items are rare and interesting. Some give bonuses, but the numbers are always small. In the same vein, you're not collecting hundreds of bonuses and penalties and figuring out which ones apply when; you either have advantage (roll twice and take the greater), disadvantage (roll twice and take the worse) or neither. Maybe, in a rare situation, there's a small bonus thrown in there somewhere.
Classes are not just balanced, but interesting. Reading through the PHB, I want to play every class (except ranger). Every class has fun new abilities at every level, in addition to paths similar to Pathfinder archetypes, which make it possible to have 2 or 3 of the same class in the party without everyone looking like clones.
With some exceptions, spells are much more balanced, without having to use 3.5's solution of reducing spell duration. You can only have one buff up at a time (for the most part), and you never have more than one 9th level spell per day. DCs are based on caster level, so 1st level spells and cantrips are useful even at level 20. Using higher level spell slots for low level spells gives you nice bonuses, and you always have damaging cantrips to fall back on which can be cast an unlimited number of times per day and do damage that scales with level. You can be a viable spellcaster who can only cast up to 5th level spells but has 9th level spell slots (multiclassing can get you into that situation).
Multiclassing is neither required like in 3.5, overly complicated like in 2nd, nor detrimental (often) like in Pathfinder. Every class except ranger is perfectly viable for 20 levels, and you can also multiclass for some interesting combinations. You can even multiclass spell-casting classes and still be viable.
The optional Feats rule is fun. You can either get +2 to an ability score, or take a feat, most of which give you three interesting abilities.
Flaws? There aren't many that I've seen. The ranger is stupidly under-powered, so don't play a ranger. It's trivial to make a rogue who can hide and sneak attack every round, which may or may not be over-powered, but it does mean it's hard to justify a non-halfling rogue. Certain spell combinations are broken (simulacrum + wish = unlimited money), but nothing a DM can't fix. Skills are nice and simple, which also makes them less interesting, because two PCs with the same level and class will always have the same bonuses to certain skills, but that won't matter unless you play a large number of 5E PCs or if there are two members of the same class in the party.
Well, there's my first post in years, and it's a biggie. I hope it helps.
