I'm going to chip in and say that D&D 4e was a great system marred by poor content. (As I understand it, and I might have heard wrong, the person who designed the core system left WotC during development, and much of the classes/powers/feats were written without his input.)

Good System:
--One resolution mechanic: Roll when you do something, try to roll high.
--One resource model: Every class has At-Will/Encounter/Daily powers. Every class gets feats at the same levels.
--Streamlined level scaling: Add half your level to most d20 rolls (no more BAB and saves scaling differently by class).
--Ritual magic: Lots of magic not useful in combat became rituals that anyone could learn with a feat.

Bad Content:
--Classes felt very same-y, because there was no variation in when they got At-Will/Encounter/Daily powers, or how many, until PHB3. Also, each class had its own selection of powers, except that many of them tended to be replicated with minimal changes for several classes (i.e. there were Fireball-like spells for several arcane classes).
--Classes got pigeon-holed into a specific role with no freedom to branch out (except maybe by multiclassing). That simplified balancing, but seriously limited character creation options.
--Powers felt very game-y, as if the designers started from the mechanics and worked backwards to come up with flavor.
--Relatedly, a lot of powers could be summarized as "I deal damage and cause some other effect." Complex and/or non-damaging powers were rare.
--Monsters' attacks, hit points, defenses, and damage scaled faster than PCs could usually keep up with, and especially at high levels, fights turned into slugfests. Attempts to fix it in later books created must-have feats that most characters couldn't do without.
--Ritual magic was too expensive and time-consuming to use regularly.

My ideal system looks a lot like "4e but with better content," and I'll add that I'm writing something in that niche...might make it into public playtesting via Foundry VTT once I finish my dissertation.