Without having read much of this thread, my main issues with 5e:
- PC choice points are too few and too frontloaded. You pick your race, class, and subclass in the first couple of levels, and then the only choices you get to make for the entire rest of your PCs life are ASI/feats (or spells, if you're playing a spells-known character, or multiclassing, if you're a masochist). This means you have a rough time developing your PC naturally in response to what happens to them as the campaign develops, and also, if you make a bad choice early you're locked into it and don't really have any way of compensating with better choices or a course change later on. On a similar matter, it's too hard to make relatively minor PC changes - learning a new skill, for instance - without a massive investment of resources (and sure, DMs can houserule downtime training etc, but it's still an issue)
- the weapon table is a mess. Rapiers are so much better than any other finesse weapon that they've become ubiquitous, a weapon being Versatile is basically useless, and this is magnified by the fact that already powerful weapon options (great weapons, ranged weapons) have powerful feats that make them even better. And classes like the rogue and barbarian have meaningless and annoying restrictions on which weapons their class features work with.
- the system is seeming allergic to permanence or lasting consequences for anything. Ease of healing is a big part of the issue, but also poison is a short-term and mostly instantaneous effect and can mostly be made trivial with a second level spell (which cuts out all sorts of race-against-time-to-find-the-antidote plots etc - disease has the same problem). And very few spells allow you to affect the game world in a long-lasting way. But for some reason exhaustion is incredibly persistent (and is allowed to cripple the most iconic barbarian subclass, but that's another issue)
- the whole design of the bard class is misconceived. They should mostly be charismatic generalists focused around support and inspiration, instead they're primary spellcasters with a couple of special rules and some bonus musical instrument proficiencies which never matter to their class abilities. This really limits the scope for developing subclasses (like the marshal...) around them.
- and while this is less a core system issue, it bugs me that there's very few downsides to class or race choices. They introduced Sunlight Sensitivity and the exhaustion penalty to Frenzy in the PHB, and these two downside features were way too onerous. Unfortunately, rather than just being more careful with balance, they've written the whole concept off to the point where we don't even have 25ft movement races any more. Surely you could apply smaller, more targeted penalties to some races which could be used as a minor balancing factor while adding flavour? Disadvantage when climbing for centaurs, or when swimming for autognomes, for instance.
- edit: oh yeah, and there really needs to be an additional skill that covers stuff like law, politics, financial management etc. Something you can roll to see how financially successful your estate is, or to remember a local law, or to manage a business, or to know who the wealthiest people in the region are (to ask for a loan, or rob/con them...) or understand the significance of a political event, or to know which of the king's officers you need to apply to for a specific favour. You end up rolling History or Investigation for everything otherwise, and that's just weird.
