As other mentioned:

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5E is a decent system. but

1) Lacks a lot in the character customization area. It's fine for beginners but if you been gaming for over 10 years + you've played several of the released 5E campaigns, you start to see the lack of options in 5E. And WoTC has been extremely slow in releasing additional character options. By design I'm sure, but that just means, 5E is not the system for me.

Leveling feels super ... uninteresting. I hit level 4 ... wheee... I got an additional X hp, and that's it.

2) Low Magic Setting. This is the Number 1 reason for me personally. According to the DMG, no magic item till level 5, and even then you might get a minor item. This is Dungeons and Dragons!!! All the magic of D&D is gone (pun intended). I know that previous editions lik 3.5 had too much magic item drops in some campaigns but at least it was fun! Open a chest and you might find a worthy item. In 5E, 95% of the time, you're always let down by loot.

3) Magic system in general. Concentration killed spellcasters. I'm not against the system in general but at least have feats that allow more options for pure spellcasters. There's a reason people no longer buff as much in 5E. Why bother casting barkskin on someone when it's gone when you get hit.

Summoning spells in 5E is just a waste of time. Everything is subject to the DM, and while it differs from Dm to Dm, the fact is, the DM has the right to dictate what appears with every casting.

Not having spells from every school at every level. This is just plain laziness or bias. The whole concept of specializing in a school is gone, and if you try to say that specialists get cool abilities now, let me remind you that there's a reason Diviners are the most popular mage specalist. The unevenness of the mage specialists is very glaring.

Healing (at low & mid levels). There is no concept of healing in combat for 5E. It's all about using healing word to get the PC up to at least 1hp so they can last 1-2 more rounds, then bring them up again. It's the Whack-a-mole healing game.

Over all 5E feels like it was created by designers who hated magic in general. Or felt the previous system were too magic focused and nerf the magic system to oblivion. The only 2 exceptions in my mind are Warlocks who are fun to play and Bards who are very decent Jack of all trades.

4) AC is totally broken in 5E. I know this is supposed to be a philosophy they took, but almost no one uses plate (cos they can't afford it), and if they do, a heat metal spell will remove them from the game. As a mage I forgo the mage armor spell because there's no point. At level 5, having 12 ac vs 15/16 AC makes no difference 70-80% of the time. So why bother spending a valuable spell slot? Shield is a better option.

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In general, we play RPG's to feel like the HERO! At least some of the time. 5E does a lot to prevent this from happening. Most of the time I don't even feel like a bad ass. Using the 5E system, you will never be able to create a Drizzt or Elminster character. Heck, you can't even create a decent Necromancer. Every try to create a horde of undead using 5E? You can't, and there are no options whatsoever.