A bit of my own background, I don't recall exactly which edition of DnD it was I played, but my first was either the original DnD or Advanced. A good chunk of my gaming after that was Second edition, though when I wasn't gaming with my brothers and found another group of friends it was largely third edition, as well as 3.5. I did not like fourth edition, and then I played Pathfinder and that became my favorite tabletop.
I didn't really like the look of 5E,but the group I'm in now seemed to really like it, so I play regardless. May not be a system I like, but I do still like gaming with my friends, so it's all good, I enjoy the time regardless of system.
All that out of the way, it's best to look at what I think the actual goal of Fifth edition is. Obviously to sell books, but when looking at it compared to other editions I think there are two things that are obvious in the design philosophy.The first is speed, they want the games to run faster. This is not my contention, but more the second reason. They want it simplified and make it easier to understand and run the game, and they succeeded with this with flying colors.
Gone are Base Attack Bonus, the three base saving throws, Spell level DC's.and skill ranks. All that is replaced by Proficiency. Virtually every modifier is determined by your Proficiency, which is based on character level, and stat modifier. Any skills you are 'Proficient' with, you add stat and proficiency to rolls. All attacks are stat plus your proficiency,saving throws are just your stat, and if your class is proficient in that stat for saving throws, you get a proficiency.
I know I am backtracking a bit here,but I'm using this to highlight the trade-off with simplifying a tabletop game, but one thing I loved in Second Edition D&D was 'Skills and Powers', one of the supplemental books. See, in second edition each stat came with a list of what it effected. For example,having an eighteen strength would give you a specific carry capacity,bonus to damage, bonus to hit, a bonus to the percent chance to break things. Skills and powers, however, allowed you to treat your Strenght as one higher for a certain selection of those, and one lower for another selection of those. It allowed for a degree o fcustomization for the system. It was a bit abuse able for classes that didn't cast spells, as you could just lower the set of abilities that affect magic and effectively lose nothing while treating your stat as bigger. That said, a bit of tweaking could fix that.
Move on the third edition and the list of abilities your stat effects were replaced by a single modifier. The higher the stat, the higher the modifier attached. This helped smooth things out a lot, and things began to run smoother. You'd see what skills were strength-based and add your strength modifier. Skill checks became your Ranks in a Skill, which were points you could allocate as you saw fit (There was an upper limit based on level though).
While I think that, overall, this was a good move for the game, there is no getting around you lost a little bit of potential customization that was available in some thing like Skills and Powers from Second edition. You had some other ways to tweak your characters that worked within the system, but a level of customization was lost all the same, and it was a bit sad.
Fifth Edition sees even greater simplification with the removal of skill ranks entirely. You have the skills you get at the beginning you are proficient in, and with a few exceptions, that's what you get. You can no longer choose to invest a couple of points in another skill so you are mildly proficient in it,even if you don't keep up on it for the whole game. It's a small thing that not everyone would do, but options become lost all the same. I could go on, but you get the point I think.
The second key issue I have is Advantage. Typically magic and specific conditions in older editions would modify your roll. If you and an ally are flanking a target, for example, you each get a +2 to your attack roll. Firing outside of your ranged weapon firing range can incur a penalty to your attack roll based on how far out the shot is. Spells have varying bonus's if used to buff allies, and all these conditions can affect the outcome of the roll. All that is gone in fifth edition and replaced with advantage. Having advantage means you roll twice and take the higher of the two.
Here is the thing, let's say your vision is impaired by a spell, you will roll at Disadvantage. Same as Advantage, only you take the lower of the two rolls. Now let's say your vision is imparted, you are backed into a corner where the walls have spiked with invisible enemies on all sides that aren't blocked by the wall, you have a rabbit wolverine in your pants gnawing at your balls, and someone has just telepathically informed you your mother just got caught but a bunch of Goblins from Goblin Slayer, you roll at disadvantage. There is, mechanically, no difference between these two scenarios. An extreme example, yes, but everything on that spectrum is treated exactly the same.
The simplification of fifth edition doesn't just limit what the player can do with their character, it limit's the kind of interesting scenarios you can have by boiling everything down to advantage and disadvantage. It makes the game really simple and easy to follow, it's really easy for new players to pick up, and it does wonderful at that goal. But the trade-off of customization is too much for me to get behind, too much of the nuance becomes lost.
I get why it's such a runaway success,and I don't begrudge anyone who plays it, but I like my systems with a bit more meat on their bones, and in the end me and this game just don't see eye to eye on a very fundamental level, one that can't really just be home-brewed away. It's a very well made system, just not one that was ever going to be able to appeal to me.