I've played 4e, and to be fair, it's pretty deserving of its reputation.
Okay, so your powers are all divided into 'per round', 'per encounter' and 'per day'. There's lots of moves that can reposition opponents (especially for Monks) so a map is pretty much required to know what the hell's going on and the whole thing just feels like someone tried to make a MMO version of D&D.
That's part of the reason why 5e went back to basics, honestly, so yeah... upsides and downsides.
As for high level games... you're not missing much. The thing about D&D is that most of your combat capability is dependent on gear, so at some point you reach a level where you could simply deal more than your max HP in damage in a single round, and it just turns into an Initiative Lottery where everything under 40 AC is a death wish. I think D&D is at its best when you stop leveling at 9-12, which is why E6 games are fun: you cap out at a level where you can get your first taste of that level of power, without having all your bases covered. Going around casting Heroism, Holy Aura
Gestalt games especially are mostly to just play a party of wrecking balls fighting other similarly beefed up monsters, but it's a kind of... damage multiplier really. Sure, you get the class abilities of two classes for the price of one, so at level 8 you could be a Paladin 8 / Sorcerer 8 with full BaB, good Fort and Will saves and adds their casting stat on top of said saves.
Personally, i was a Paladin/Cleric at the time, with prestige classes in Knight of the Raven (
https://dndtools.net/classes/knight-of-the-raven/ ) on the Cleric side and Cavalier (
https://dndtools.net/classes/cavalier/ ) on the Paladin side. One of my favorite tricks was casting Anti-magic Field (my mount was physically there, so unaffected) and then charge in, using the combination of a x5 multiplier (x4 actually, still needed a few levels to max out) on a lance charge against an opponent... that had all their protective magics suppressed because Anti-magic Field.
Ingame i was the leader of a Paladin Order that was sent to arrest the party, but she had her doubts that they were truly guilty (they agreed to have her cast a Geas on one of them to 'not lie'), so she made an agreement to tag along and see for herself what they were up to. She focused on teleportation magic too, so her tactics consisted of sending her raven to scout, studying the target location through the eyes of her raven, buff up her squad and then Greater Teleport everyone through to completely blindside her enemies. Never got to show it since the party decided to negotiate instead of run, but yeah, that was a nice setup to have.