The words I always use to describe 4e are streamlined, focused, and transparent.
Streamlined attempts to remove things from the game that just don't really matter to the gameplay loop. This was a huge divide for some. While Gygax claimed the tracking of time was important to the game, it's been my experience that time is an abstract construct that serves the narrative. Look at spell durations for example. Rounds per level? We track that. Minutes per level? Well it lasts for one fight, but if I have a power that lasts for 7 minutes, whether or not it's going to last into another combat is rarely a question.
It doesn't, even if that's actually 70 rounds of combat, lol!
Ten minutes per level gets a bit wonky, but generally you can squeeze a second fight out of it.
And hours per level is usually "all day" and not worth tracking.
4e was like "so what if durations were literally, 1 turn, encounter, or "save ends" which gives a variable duration?". What if "short rest" was a non-intrusive amount of time that you can assume it happens 99% of the time?
What if instead of spell slots and x per day abilities, we just said "you can use this move as much as you want, this is 1/encounter, and this is a big 1/day ability for tough battles?".
What if we don't worry about geometry and say "you move x squares on a grid"? Which really is no different than AD&D movement being described in inches on a map?
What if minor foes that can generally be dispatched in a single attack LITERALLY can be dispatched in a single attack?
Transparent, on the other hand, says things like: What if powers, rather than lots of descriptive text, just have a quick line of flavor text, and then they do exactly what they say they do? What if monster knowledge is an accepted part of the game, and players have a chance to know what an enemy will do in an encounter, allowing them to make strategic, informed decisions instead of every monster being "surprise, look what I can do!".
What if there are no "secret" or "hidden" rules and we explain the nuts and bolts of how and why the game functions?
Focused, then, narrows down the game to the part that requires the most adjudication. Combat and character options relating to combat, historically, take up the bulk of rulebooks. So let's make sure that part works, and then handwave the rest, allowing people to quickly move past non-combat encounters, since that seems to be what a lot of groups are doing?
This right here, I understand the issue with: I once tried running an old school dungeon in 4e, and it was a train wreck because if it's not swinging a sword or using a power, it's a skill challenge, lol. Oh sure, utility powers and feats not meant for combat exist, but nobody takes those, including most rituals, because you know, as a player, you're going to get into a fight.
You don't know, as a player, if you're going to need an invisible ladder of force once every 5 minutes. And even if you have it, at best it's going to be equal to an automatic success in a skill challenge, if the DM agrees it's even useful!
It's right here where I realized that 4e was fun to play, easy to DM combats for and create combat encounters, but was literally not meant for other kinds of play.
Of course, this is nothing new, I mean, even in 5e, social and exploration tiers of play are virtually non-existent. But 4e came out and said it, and that rubs a lot of people the wrong way.