For the record all the build things (d/w warriors, ranged rogue) Reaver listed did eventually get added into 4e. It did take time.
4e Standardized and balanced D&D. The only edition where casters didn't eventually run the table in terms of balance.
It's also simplified a lot of things. Residium (Magic Item Creation), Rituals, Tiered Currency, even counting squares on the map, monsters and encounter design etc. Even reward progression was built into 4e. It accomplished what had been wanted from the early days of D&D, the ability make all character transferable.
It's addition of Skill challenges, scale able monsters, easy encounter design all made 4e a well built game. Monster design of 4e is particularly notable. Monster had category types that help modify their stats. Soldiers, Brutes, Lurkers, Minions, Solos. 5e's monster are all built following the 4e Brute rules, (Low Defenses, lots of Hit points).
Even combat in 4e was improved, Bloody (Being at or below half health), gave designed many more options. Monster often had effects that were trigger when they were bloody. This sort of created sort of scripted type encounters, which added interesting things to the 4e table.
Now of course nothing is perfect and ss with every Edition of D&D 4th did have trouble with scaling, as good as they were, C&C mechanics did eventually largely trivialize the epic tier (level 20+). (That said, Spells do that ever other edition of d&d generally around 11th level. So We got 9 more levels then we did previously).
The party of 4e Ended with the release of Essentials books. This tragic mistake was done to address the complaints, of those who just didn't like that you had multiple powers and had to make choices. This put the edition to into a book bloat death spiral, as they no longer continued to push the developed that most of it's fan's liked.
Another major failing of 4e, was how easy it was to build a very ineffective characters. This was a very serious issue. It required good level of system understanding across both class, gear, feats, and stats to make a truly effective builds. Further to this, was power sources, 4e had power sources, for example, Arcane, Divine, Martial, and then a complete set of classes for each power source. (Class roles were Leader (Healer), Striker (DPS), Defender (Tank), and Controller.) While it wasn't necessary to build unified power sources it helped increased party synergy fairly well. Something that was fairly rare.
Just to help illustrate, what I mean when I say it was easy to build ineffective characters, towards the end of 4e, a lot optimization was done around making characters that could carry a whole party through an adventure, to add insult to injury it was certainly possible to achieve that goal.
There was also no follower or strong hold systems in 4e. Something many old school players really liked.