So my thought on why I feel burned out and dissatisfied with D&D 5E has to do with these elements:
1. I have run D&D for four decades now, pretty consistently. The golden era of my enjoying the game has passed, but I still relish the idea of playing a game that gives me that feeling and excitement and discovery I got out of so many games in the past. For reasons I am mulling over D&D 5E really has a hard time doing this for me.
2. D&D 5E is a complex game by any standard but a simpler game than many others such as Pathfinder. I don't really think the issue is complexity, though....or otherwise why would I find Cypher System so appealing? I think that in pursuit of that novel experience I am finding other games (like Cypher) are filling that role better than D&D now.
3. More complex games can accomplish this because they add a layer of fiddly bits that can be almost ritualistic in application. Half of playing Pathfinder or Starfinder is trying to figure our and master the mechanics, for example...and you can spend years playing without achieving full mastery. For that reason they can have an appeal in this case because they add elements through mechanical bits that make the experience more complex and interesting to the old jaded gamer soul. This is a severe case of YMMV though because complexity is not appreciated by everyone (not even me at times). When 5E came out I was really burned out on running high level Pathfinder, for example, and welcomed a change of pace.
4. D&D has a lot of -isms. We know what these all are, but often times when I run games these -isms (also known as tropes) become force of habit. Unfortunately it can be difficult for me to chop my way through the forest for the trees, so these -isms/tropes can be difficult to scrub from a D&D game (and still have it deliver an experience that justifies playing D&D over another different game). Examples of these tropes include:
--dungeons, in all their grid mapped glory as a center point of any game. I have known I am burned out on this trope for a long time, but often it is still a core conceit of many scenarios and what the game supports better than almost anything else.
--Heroic scales. I really crave more realism, with heroes who need to struggle, or who need to manage resources carefully and maybe find escape or flight from a menace to be a sound decision. This is very hard to accomplish in D&D 5th edition without making the game feel unfair. Games that do this very well I feel include BRP, Runequest and Mythras (all related systems). I had a similar complaint to 4th edition D&D too, which was much worse in terms of its "realism" level. D&D has never been about realism, of course....I am not asking it to change for me, just stating I've changed and can't dig it as well as I once did. The heroic scale of D&D has been with it since the beginning (if you level high enough, of course).
--There's a lot of little stuff that is just so very, very D&D: let's make camp; wandering monsters; treasure tables; traps; putting weight on the need to slaughter monsters of CR appropriate levels over a long time to earn XP to level up (admittedly 5E has alternative rules if you want); magic works THIS way; here's seven thousand monster tomes with all the same monsters, over and over again; here are the same classes; here are the same modules and settings, but now in 5E edition; here's the same core conceits of world design, with the same broad range of options; here are the same magic items; these are all "D&D" and frankly all fun and exciting....unless you've run possibly as many as 2,000 or more games (probably more) over five or six editions.
5. Tired of the results of Big Hit Points, Big Damage and bounded accuracy. I liked these design points but over several years they make the game feel less interesting and more generally predictable. It is rare that a D&D game surprises me anymore from a numbers viewpoint.
6. Skill based system. This is an odd one because I like Cypher and it's not skill heavy, but D&D up through 3.X and Pathfinder had a very robust skill mechanic which I miss. It allowed for a lot fo nuance within the scope of a level and class based system. A skill system isn't a deal breaker, but for D&D style play I would like to see something more robust.
7. Story Mechanics. A year ago I would have not thought this was important, but it's become increasingly relevant to my gaming habits, especially after falling for Cypher System. I can mod these in to D&D, and indeed I've adapted the Gamestery Plot cards in to D&D for years now, but I'd love to add in other elements similar to GM and player Intrusion mechanics. XP mechanics for special effects or temporary gains similar to Cypher and other pieces could make a more robust experience....although how to make it still feel like D&D is a question to be answered.
With that said, I think D&D 5E is a fine system, but it may be the first time I wore out on an edition of D&D before a new edition was on the horizon. Worse yet I think I've been on a slow burn with my discontent for over a year now, and it took experimenting with other offbeat games like Cypher, Numenera and Genesys to realize that maybe I just need to give D&D a break for a while, possibly like a really long while.
One very important side note is that part of my issue is changing times: I have less time (a lot less time) to spend prepping and even thinking and reading about gaming now than ever before in my life, due to family and work. As a result, my free time to properly enjoy RPGs is at an all time low, and with what time I have I need systems that I can handle in that limited time, and which feel like I'm making good use of that free time I do have. Ironically the main reason I am enjoying Starfinder is because it redesigned the 3.X mechanics to be more streamlined for the GM, and an amazing person made www.sfrpgtools.com, without which I'd never have a chance to design statblocks every other week.
This also explains my ongoing fascination for Cypher System, a game designed to make the most out of a lazy or time-deficient GM's life to run games. With Cypher System I can world build and plot design without worrying much about mechanics....Cypher is built with my style of GMing in mind. It is also a game which can comfortably run without ever leaning in toward violence as a storytelling medium, and this may sound wackadoodles to some, but I really enjoy games these days where violence is the threat of last resort rather than the core conceit. Don't know why, I just do.