It’s a (relatively) crunchy, and very wordy system - the core concepts are easy to understand, but there’s so many statuses that all interact slightly differently with actions, and spells in particular, that make it dreadful to actually follow to the rules as written if that’s what you want to do (and that should not be a problem with a ruleset!!)
The core mechanic is one I’m not much enthused by: it’s a very binary pass or fail system, you either succeed or you don’t. A lot of other games have a “you do really well!”, “you succeed, but at a cost”, etc. or a degrees of success/failure system that feels much more gradient, without being even slightly more complex for it.
Various mechanics interact with this in a way that isn’t particularly exciting; armour doesn’t protect your character when they’re hit, it simply makes the binary success/fail chance enemies have to hit you more likely to be a fail, spells will either simply not work or do a bit of damage and have no further effects instead, etc.
I find that, especially, hampers a lot of the actual game. It tries to be a tactics game; yet its tactics revolve around a binary pass/fail check. It also tries to be a vehicle for dramatic storytelling, yet all of the social-related challenges you might face are, in fact, also just a pass/fail!
I personally find that a lot of the fun had with D&D5e is often despite the ruleset, rather than the ruleset enabling it. It’s a system with a medium amount of crunch that tries to be a lot of things, but due to its core mechanics being a hard pass/fail, kind of fails at being both a Tactical Battle Simulator™ and a Dramatic Storytelling Device™
I think that, despite not being as easy a system as people think it is (there’s a many games easier to understand!), 5e has a fantastic amount of resources available that help people get into it and learn the system, as well as the general ideas and concepts of the whole tabletop roleplaying game thing. There’s a lot of player classes that are pretty well designed and that are, more importantly, fun to play and good for inspiring the actual character! Warlocks, Clerics, and Paladins being as they are really want to make you think about your character and their connection with [source of their powers], and something like an Aberrant Mind Sorcerer is stupidly cool.
Any other game. I dab and walk off stage. But it mostly depends on what I want my game to be about.
I want to do something heist-y? Blades in the Dark.
Investigating mysterious stuff? Call of C’thulu, Delta Green, GUMSHOE engine/system-based RPGs
Sandworms? Dune
Powered by the Apocalypse-games (like the Avatar rpg, although I’ve never been much into Avatar) or something like Ironsworn for a narrative-driven “yes, and…” sort of game, where failing means “falling forward” rather than nothing happening, etc.
Warhammer Fantasy RP 2e/4e for investigation-y things in a medieval setting (Zweihander applies too, but I hate the author so grr). They’re systems that work well for investigation-based campaigns, but also social/court intrigue kind of things, or something combat-heavy, depending on the party’s made up of (I would go into battle with soldier, knight etc careers but probably not with my cobbler and tax collector…)
For heroic combat-isms and dungeoneering I really like something like Cubicle 7’s D6 system - it’s currently only in Age of Sigmar Soulbound, but they’re releasing it as its own thing, with their own setting, somewhere either this year or early next year.
5th edition Vampire the Masquerade is very setting-specific by default, but it’s great for personal horror and a lot of social encounters - its combat mechanics are shared with a social combat mechanic that feels really great to use for particularly dramatic social exchanges!
gee whiz that’s a big post, and that’s me being brief!!
tl;dr i think 5e’s core mechanics are pretty bad, and 5e tries to be many things at the same time, but it doesn’t do so particularly well when you’ve played games that focus on those specific things. Since the usual d&d campaign will primarily focus on one of these things, I find it far better to pick a system tailored to it!
I’m not saying people are having WrongFun, but I do think expanding one’s horizons allows for More Fun.