Some incoherent thoughts:
One of the things I noticed over time how the implied setting - which was started in the "Worlds & Monsters" books before the PHB was even released - really appealed to me. It used lots of components I knew from D&D 3E (my first D&D edition) ,put a new spin on it, and created something very coherent, but without being too detailed. You had plenty of room to tinker and make it your own story.
Playing it, it was surprisingly easy to DM. The monsters were more simply to both create and play, and handling a combat encounter was much easier than I knew from other games (Shadorun 3E, D&D 3E and Monte Cook's Arcane Unearthed) being the primary games I had DMed or GMed before).
That didn't make creating the "story" of the campaign or adventures easier, but it freed a lot cognitive workload to focus more on that.
I also noticed that I barely knew the powers the player characters had, because the rules framework made it easy to adjudicate everything. And it's fun to be surprised by some of the powers that existed, and seeing how an enemy would end up in a very bad situation, or how a player could escape a similar bad situation with tricks I didn't knew (or at least didn't remember) he had.
The Warl*** classes were probably my favorites. Warlock had great flavour (or flavours, depending on your patron choice), and Warlord was just something I had always been missing. It seemed that very often the "guy with the sword" was the party leader in fiction, and finally there was a fighter type class that could actually pull it off. And on top of that, you could also do the "lazy" Warlord, which was a very interesting concept that I wish more games were offering.
Overall, as a player, I liked that there was more focus on character play than character building. It's not that you didn't plan your character and carefully selected your powers, but the fights felt rather different because positioning and the environment mattered more, and you had more ways to affect it, too. Especially the force movement powers and zone creation abilities tended to create a more dynamic battlefield. You could really work towards subgoals in combat.
I enjoyed the Mark mechanics and the related "punishment" mechanics. It felt better to not just have the enemy be "mind-controlled" by some aggro mechanic like common in computer games, and it also meant interesting choices to be made - do I take the Defender's punishment, or do I focus on a more vulnerable but dangerous target. It's not always clear what is best, and either way, the Defender's presence will matter.
I think the healing mechanics also played into these dynamics. You could be reduced to bloodied (brilliant invention) in very few turns, increasing the tension, but a healer (or sometimes a non-healer ,including yourself) could allow you to spend a healing surge (plus some extra healing) and really avoid you acutally dropping (and potentially losing turn of actions). It can be a bit of a Yo-Yo effect, but only in your hit point values, your character is (usually) not actually dropping. Without such a mechanic, it's really just a matter of straightforward attrition, which tends to start boring and in the end gets a nailbiter, but you can't do much about it once it happens. With the healing surge mechanics, you kept some of the attrition (but usually across multiple enounters), but keep forcing people to act and react, and it happens not just at the end of combat, but the entire time.
While it did also slow down play, the many interrupts, reactions and opportunity attack abilities meant that even when it's not your turn, you need to keep track of what was going on, keeping you more engaged. (Even though sometimes I wish these abilites were more streamlined).
Overall that is why I think I agree with many that D&D 4 really benefits from bigger set piece combats and not much from a series of lower difficulty encounters. If you have a challenging fight 2-4 levels over party levels (at paragon or epic tier maybe more), because all this really starts to matter and work together to create a very interesting combat, even if takes time to play.
I think that kinda benefits my gaming group. We're playing online with Maptools or Foundry, and we only have 2 hours or so of gameplay a week, with many multi-week-long breaks due to vacations, ilnesses or what not.
Low difficulty fights tend to be not that exciting, and having to track minute resources like spells/powers per day and per encounter tend to be difficult and take time away from the storytelling and character play.
Paragon Paths and Epic Destinies were cool and often inspiring elements. When I read the words "Once per day, when you die" for a special ability I really knew Epic meant Epic.
D&D Essentials had some interesting ideas, like trying to create "simple" versions of classes that avoided the AEDU concept to appeal to gamers that didn't enjoy them. It probably needed more finetuning, but ultimately it came too late.
Still part of me thought it would be fun if they had started with an Essentials like approach, or at least created some mini-starter that would use Dwarf and Elf as "classes", and then later in a full game reveal that they are just a 4E Dwarven Slayer and an Elven Two-Weapon Ranger or Swordmage in a trench-coat.
The digital tools were a great addition, especially since they incorporated errata, but primarily because they made managing all the supplements so much easier. And they made NPC/monster building and adjusting so much easier.
I wasn't a big fan of the move to the online version of the tools since the character builder became less responsive, and the monster builder was never quite done. Kept using the offline monster builder for a long time, even if it meant having to readjust some stats to the new monster math. But the worst aspect of that was of course that when the digital support went offline, the tools were gone.
I actually used 4E as basis to create a Star Wars rule system, which allowed me to run another campain over several years.
I still am a bit creatively burned out after the years of DMing (even though we share DM duties in my current group), but one of the serious contenders is a 4E based campaign using the Diamond Throne setting.
