I'm not bashing anyone's enjoyment of 5e. I know I'm in the minority for not liking it. I can only say what doesn't work for me.
All of these points are IMO....
1) Poorly designed adventures
2) Weak campaign settings
And it's difficult to build my own of either because of...
3) Badly implemented CR system
4) Inconsistent monster design rules
5) Lack of guidance in how to reward characters (nothing to spend gold on, no magic item economy, etc)
And things get stale when...
6) There are clearly better weapons, ability scores, and ancestry options - and new options are stagnant
7) Every party feels about the same
8) Tactics don't really matter when the rules don't reward smart play ... and it's not really required anyway
Because the monsters...
9) Don't have enough actions to threaten a party
10) Don't have high enough damage potential to threaten (low damage, low attack bonus, easy accessibility of PC healing)
11) Conversely have too many HP to die quickly, and not enough to require the party to work together (mostly because there's little threat - see Point 10)
12) Or have breath weapons that can TPK a party ("Rocks fall, everyone dies")
And the characters are limited by...
13) The game ending at 10th level (because there's no guidance on what to do or published content)
14) They can already stomp high level baddies at mid tier
15) There's no point to adventure because there are no rewards (see Point 5)