Hi, I quit realm like well over a year ago (about 1.5 years now) and I had a number of things that caused me to leave; more of a "death by a thousand cuts" sort of thing.

TL;DR:
My slow burnout is based off of the game incentivizing content that I don't want to do. Along with the direction of the game just being overall more serious (even though there are... outliers). Mostly small things but like a lot of small things.
Who am I?

 I'm just a dude who got semi-popular on YouTube in the realm sphere, otherwise my opinion is just that of another player's. I did get white-star (before the fame change and kept it after the fame change), and I have well over 9,000 hours in Realm. I actively helped anyone who cared to ask for help, along with being a player skilled enough to carry basically any group that I was in (with just about any gear). I am also one of the handful of players who are able to solo a void and have solo'd a lot of the more difficult content in realm. So when I say that I was burned out over a long long period of time, just know that things that I mention may be similar reasons for other people.

Exaltation Grind:

My main thing was that I felt like the game was trying to force me to do things if I wanted to have the "best time" afterwards.
Examples are like how my favorite class was knight, but if I wanted the best time for playing knight, I would have to exalt a bunch of other classes to have the best time. Very time consuming and it actively encouraged the behavior of not doing what I liked to do. "Oh you want a better time for knight? Go play all of the sword characters and exalt them all. Want an even better time? Exalt all heavy armors. Want an even BETTER time?? Exalt everything for more loot."

Do YOU want to play Necromancer?? (Maybe with recent updates, but back then? You incentivize that??)

You could argue that I didn't have to, and you're right that I didn't have to, but if the game is designed in a way that incentivizes the behavior, then it should not be ignored.

Seasonal and Shinies:

In the same vein as doing things I don't want to do, why are shinies locked to seasonal? IDK if they still are, but you mean to tell me that we spend all of this time and effort into making characters that will last a long time, only to need to create a new character every season in order to get the most out of the game? I want to play the characters that I have that I have already built a lot of sentiment with.

Speaking of playing characters I want to play, there is an idea in the pokemon world called a nuzlocke, where basically you have to introduce the idea of perma-death and limited pokemon per run, with an added thing to name each pokemon so that you get more attached to the pokemon. It's a very exciting challenge and keeps players interested, who otherwise wouldn't be into pokemon, but the reason why I am bringing this up is that the whole appeal of realm to a lot of people is that you build up a character from nothing and just see how far you can take it. That's why PPEs are big content farming for a lot of people. However, if you character is still alive by the time the season rolls over, you basically are forced to reset your progress if you want to enjoy the "benefits" of seasonal (again, why tf are shinies locked to seasonal).

A lot of the community was hoping for the seasonal mode to be a ppe mode, but with it being able to be pay-to-win'd and having benefits like events and things like that, it just feels like a "mode you have to play" for certain content rather than a ppe mode.

Game "Seriousness":

There were a lot of "goofier" strategies that I absolutely loved. I enjoy gimmick strats a lot. Older realm had a lot of things that were just super gimmicky, like cshield, EPing (that one is still in the game but largely designed against it), and just a lot of other stuff that I can't think of (remember that I quit well over a year ago by now). Not only were those strategies designed against, some were even just "reworked" removed from the game. I love these stupid dumb strategies, and instead of being able to play it, the game was incentivizing that I did anything else.

The game balance and design being catered more towards the more "serious" side, like removing gimmicks, making items more balanced to each other as side-grades (horizonal progression instead of vertical progression), and classes more "all around" instead of dedicated roles like support/tank/damage roles that other MMOs have. These things all make the game more user friendly, and even more enticing to solo players who want to be able to deal lots of damage on non-dps classes. However, cheating in realm has never gone away, and so seeing other people basically have no consequences SUCKS even though the game is being designed in a way that should punish bad gameplay (shoutout perma-death)

Cheating:

I will be far from the last person who complains about this, but how are there still this many cheats in the game?
I won't go too much into this because Niegil already posted a big list of cheats that are active, but like bruh.

Bots in nexus and like... anywhere:

I like having my chat turned on because I like being social and teaching people how to play the game. However, before I quit realm I basically turned off chat and dm's because I was so tired of having to constantly mute bots and being dm'd things like to visit stupid sites that were against ToS.

It made me feel way more lonely playing realm than ever.

Loading Times and Crashing:

I don't know if this is an issue for a lot of other people, but sometimes updates make it so that loading into anything is a gamble. Loading into the vault sometimes took minutes at a time (and that was basically a required loading time), randomly DC from things, inventories being rolled back for no reason when you go between areas, and just stuff that makes no sense.
People can only be excited about dungeons only to dc while loading so many times.

When people talk about "quality of life," game connection and fluidity should also be on that list.

Accessibility:

I'm sure that the realm rework addressed this greatly, but I did not like the idea that certain dungeons, when you wanted to do plenty of them, needed to be done in discords or similar groups. I remember a time where I was looking around for a Sentry because I wanted to solo a halls, only to be hunting for hours for a single one. I was also someone who solo cleared down realms because I was able and couldn't find anything, so I was trying to spawn some by brute force.

Also cheaters make things like doing O3s super annoying because there were phases that were designed to intentionally not shoot, only for cheaters to just shoot because nothing mattered to them, since they could just dodge everything and have zero consequences.

okay anyways, I have other stuff to do but that should be a decent amount of stuff to chew on. Shoutout Niegil for saying "I want comments" only to receive essays by some community members