Keywords are very much an example of system mastery design. They're great for experienced players who know the game, but hard for newcomers and casual players who may memorise the rules less thoroughly.
It is not about memorizing rules. There aren't rules for these keywords, aside one very brief section in the PHB that is about half a page (and which really doesn't say much that is important). It is about being able to see plainly from the description of your Frost Blade that it does indeed add the COLD keyword and damage type to your MBA and thus interacts with your Lasting Frost feat. This is something that in 5e requires parsing dense text of 2 different game elements and then hoping that it isn't worded so vaguely that you still have to consult the DM.
Your THAC0 goes up, but the AC of enemies doesn't go up at a matching rate. It barely goes up at all.
This is patently untrue. Higher level monsters almost universally have better ACs. Particularly in 2e where monsters with more than 6-8 hit dice are MUCH more powerful, and the 1e quirk of repeating 20 5 times on the attack matrix doesn't translate to THAC0 (meaning that negative ACs are MUCH more effective than in the older edition). I've done the math, even 1e has pretty much the same overall rate of advance in bonus and defenses that 4e has. Albeit things start off a bit worse for the PCs, usually needing a 14 or so to hit, and slowly progress to where they can often hit on a 10 or maybe even an 8 if they're not taking on a top-rung enemy (but forget it if you run into demons or something like that).
And there's not the same level based tiers for monsters; very often published adventures threw lots of low level monsters at you for attrition.
Not very often. And 4e can do the same with both minions and simply standard monsters (which are only a modest threat really, particularly if they're something like level - 1 or 2). AD&D really is hard to compare in another way. Hit Dice are not really a very good measure of threat. A hill giant in AD&D could be offed by 3rd level PCs with care, heck it could have 12 hit points! Threat has more to do with special abilities/attacks than with raw hit points and such. 4e moved to a more hit point (and thus level) centered mechanic, with very few things having severe effects that aren't reflected primarily in hit points. This is why the 4-6 hit die drow in D1 and D2 are pretty deadly, because they come with lots of poison and the ability to hit and run. They're no match in face-to-face combat to 12th level PCs.
And in terms of skills, you only get better, both in terms of proficiencies and thieves' skills.
And the whole point of bounded accuracy in 5e is that you don't fall behind. You don't always encounter level appropriate locks that you always have a roughly 55% chance of disarming provided you always sink skill ranks into the check unless you find a way to optimise...
You don't 'always encounter' such things in 4e either. You are just very much likely to adventure in areas that are thematically, and thus level, appropriate to your characters. In those places most things are possible to overcome with modest luck and a little ingenuity. Its perfectly possible and entirely appropriate and within the rules, for a 4e DM to stick a 20th level lock in front of your level 6 party. They won't get past it, at least not by picking it. Maybe that's the point! The beauty of this kind of setup is, it puts things clearly in the adventure designer's court. Its clear what everything will mean and do within the adventure when its played.
Every person I have talked with or listened to who had a campaign run from 1-30 says that epic falls apart as the encounter rules collapse and PCs become broken.
Well, now you have talked to one who has the opposite experience! I'm far from alone, there are dozens of posters on EnWorld who will happily tell you the same thing. Not to mention at least 3 groups of players I ran through campaigns during my 4e GMing.
I believe that people had a problem in that they attempted to play epic as if it was just heroic with bigger numbers, AND IT IS NOT. Its a very different game which plays quite differently in a thematic and dramatic sense. Numbers-wise it actually IS pretty close. The thing is, Epic PCs have vastly more options at their disposal, and have had 30 levels to generate synergies and tactics out of the various options they have available to them. No bog standard of templatized encounter design is going to counteract that. You have to read more into things. Epic scenarios need to be cast in epic terms.
There aren't 5 standard monsters worth of opponents in a level 30 encounter, designed to the 'Commander and Troops' template. Instead its Orcus demon prince of the undead with 100 level 30 minion ghouls, a level 29 elite lieutenant, and a bunch of nasty terrain/traps to make the PCs lives hell, combined with some sort of nasty time constraint, plot twist, etc. to turn things INTERESTING! This was a hard lesson for people to learn, and WotC reworked epic monsters in MM3 and MV to try to make it easier on DMs.
It is perfectly possible to build kick-ass Epic encounters though, even using mostly stock MM1/2 monsters. I'd say it is still 50x easier than making an equivalent 3.x encounter. 5e is kind of in the middle. They clearly adopted SOME lessons from 4e, but backslid on other things for whatever reasons (I'm sorry, but the whole spells for monsters thing just sucks).
