Disclosure: I really like 5e. When 3e came out, I mostly stuck with AD&D. When 4e came out, I didn't even want to touch it and stayed with AD&D. 5e? It brought me back. Overall, I think the design team deserves a ton of credit for designing a great game. I really want to stress how even though I have a gripe with it, doesn't mean I think it's trash or a bad game. I'm not "hating" on it.
But I think I can finally place my finger on what I like about it the least, rather than a general feeling. The saving throw thread helped clarify my thoughts a bit now that I've actually given more thought to it (I've always just tried to focus on the good parts rather than spend time thinking about what I didn't like. I mean, no game is perfect, right?)
My general feeling that I didn't like? The overall less-than-lethal changes made to monsters. That's a pretty well known gripe from folks. How in most cases, you get multiple save opportunities before something really bad happens, how green slimes are completely neutered, how poison is now just a little bit of damage and not a real threat like it was before, how there is no level drain, or instant petrification, rust monsters are neutered, etc. A green slime scared the hell out of you in AD&D. 5e? Meh.
Note: I am not saying save or die was a good thing, or it was better, or that anyone who hates save or die is a bad person who beats puppies.
Upon deeper thought: It wasn't save or die or really suck that I miss, but the secondary effect of it. I.e., players were extremely cautious and genuinely careful when going out on an adventure or discovering an enemy. You approached a gaze monster totally different than you did a rust monster, and approached the undead completely different from that, and same with a venomous creature or trap, etc. You got your supplies, did your research, prepped your spells, all for things to help mitigate poison, or disease, or extra weapons (because the slime or rust monster destroyed your old ones). Scrolls because one of your most sought after magic items.
After all this, how does this translate into my gripe for 5e? My biggest gripe isn't that they don't have save or die, but that the design seems to have placed all of its eggs into the HP basket. It all seems to be about HP mitigation and attrition, rather than trying to prevent individual effects (like disease, poison, petrification, paralyzation, item destruction, etc). And that seems to lead to players approaching battles with little variation tactics. Just try to inflict as much HP as damage as possible and don't worry about getting poisoned, petrified, etc. As long as you had HP left, you will never fail.
For example, an attack by a cockatrice in 5e gave you two saving throws at an easy DC, and if you failed both of those, you were only petrified for 24 hours. So you could afford to be much more aggressive and approach combat more traditionally. Your melee types weren't nearly as scared to engage in melee as they were in AD&D, where if you failed once, you were screwed forever. So in 1e, tactics were different.
Note 2: I"m not saying tactics aren't used in 5e, just that they aren't as much of a focus because the risk of failing a save is less.
Note 3: I also understand that DMs can always change things, but I'm talking about out of the box
Essentially, really bad lingering effects are much more rare in 5e than AD&D, and lack of lingering effects changes how people approach encounters. And to me, and in my experience, it makes the battles feel a bit too similar. I found myself missing the party planning phase, and the tactics phase. The importance of prepping spells other than combat spells. The importance of getting antidotes, and protection scrolls. The importance of realizing that many battle are best not fought at all, but avoided. With 5e, it seems like all you really have to worry about is managing HP. Even most bad conditions were ended after a round or two (since you get to keep rerolling new saves every round).
So how do I think this could be addressed in the 5e framework? I think 5e already has a mechanic that most players dread: exhaustion. I don't think bringing back level draining is the answer (most people loathe it). But I think there is room to diversify the hazards a PC can face long term other than HP loss (and even that is short term). Everything in 5e seems to reset on a long rest. Boo! lol. I think exhaustion should be utilized more often, and can be used to reflect things like disease, or the effects of potent poison, or even gaining a level of exhaustion if you've been raised/revivafied/healed from 0 hp. Change the life force draining ability of undead that brought the fear of God(s) into the PCs to inflict levels of exhaustion as a way to reflect the life draining aspect. Metagaming is always going to be part of the game (everyone knows to use fire against the troll), so bring back the fear of the undead. And slimes. And other monsters. Make trap disarming a suspenseful event again.
Anyway, those are my thoughts, and what I find is the weakest part of an otherwise great game. I am totally open to hear others' ideas of how to diversify dangers and hazards to PCs in 5e. Help get out of the "HP are everything" mindset.