Level cap in a vacuum means nothing. Of course there has to be a level cap in the core game, even a game that says "there is no level cap" has an implicit limit given by the amount of material in the book, and once your PC has all the spells, feats, and abilities, you can only inflate numbers. But even numbers (and their cap) don't matter that much in absolute terms but only in relative terms with the monsters you're fighting or the DC of challenges.
OTOH, once you have a certain amount of material designed for the PC, the level cap in the core is important because it affects the spread of such material. For instance, if you have to accomodate spells of different effects magnitudes (e.g. from Magic Missile to Meteor Swarm) across all levels in core, having 20 levels instead of 30 has an effect on how many spell levels you need and/or how often do you give a class access to the next spell level. And then, arrange the spells into too few spell levels and you might have too large differences between spells of the same spell level, while on the other hand arrange them into too many spell levels and you might have too few spells per spell level which means lesser character variety (since at each spell level there are less choices, and all PC will choose mostly the same spells).
I don't care how many levels exactly they end up with, I only care that this level cap is chosen properly so that the designs of all the other stuff related to levels (from bounded accuracy to spells to skill success rate to character variety) goes well with it.
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Second, about TIERS... I really believe they should need some serious re-thinking.
If they are going with tiers in 5e, they should really avoid reducing them to stupid labels that mean nothing. Really, what is the difference between "heroic", "epic", "legendary" or "paragon"? Check out the following definitions:
http://en.wiktionary.org/wiki/hero
http://en.wiktionary.org/wiki/paragon
http://en.wiktionary.org/wiki/epic
http://en.wiktionary.org/wiki/legend
They all mean the same thing. That "epic" more than "legend" more than "heroic" is just D&D jargon we got used to and carries some habits from previous edition, but it doesn't mean anything at all outside 3e/4e.
If you want to use these labels, go ahead, but what really matters is what do they have in mind in terms of how the game changes. If it's just numbers inflating, the game doesn't change. If just names change, and you get "talents" instead of "feats", the game doesn't change unless the effects of those are significantly different. If only monster descriptions change, the game still doesn't change (you can take an orc and call it Orcus and say it rules a kingdom of the afterlife... but if you fight it just like an orc, the game hasn't changed!).
One important distinction that the designers should make is between
adventures and
world interaction. Although the 2 are related, they are NOT the same thing.
To explain, let me make some examples. Take the president of a nation and compare it with a karate world champion. Who would be higher level in D&D? The game is first about adventures, thus the karate champion would be higher level. For what we know, the president could be 1st level or less, it could be old, frail and completely unfit and unable for adventuring.
But we also want that sort of stuff in a RPG, at least some of us want it. It's just that if you hard-code powers for world interaction into the game, you get weird and squinted results. It's great if D&D supports that, but it must do so in a way that's not hard-coded into levels (it can be so as an example, but it needs to room to freely adjust it). Thus IMHO this sort of stuff should stay OUT of the concept of tiers, or alternatively it should be arrangeable freely from one tier to the other... because if your PCs just go to bigger dungeons at high levels, there is no point in forcing them to forge a kingdom, just like a karate champion does not become a president. And on the other hand, there is no point in requiring a black belt from someone to elect him president i.e. no point in forcing a minimum level for a PC/NPC to become the king. At the very least, it's campaign-dependent.
So if we want tiers, what should they be based upon?
IMHO tiers should be based on strong game-changing character capabilities during adventures, such as for example:
- seeing in the dark -> rendering illumination not an issue
- charms/compulsions -> rendering social interaction not an issue
- invisibility/undetectability -> rendering stealth not an issue
- flying -> rendering terrain not an issue
- teleportation -> rendering distance not an issue
- going back in time -> rendering causality not an issue
there are more like these of course. Now THESE are the kind of things that really CHANGE how adventures work and feel, in all three pillars social/exploration/combat!! If tiers should be in the game, they should be based on which ones of these the PC having access to and their degree of applicability (e.g. how often, for how long, how reliably, at what cost...).
Just my 2cp.