My concern comes in as both a player and a DM. While I'm a fan of rule-light RPGs where GM rulings are a frequent and expected part of the experience, most of those work best when, "The GM rules..." is an explicit part of the rules. An example in 5e D&D is DCs for ability checks. It just says the DM should assign what makes sense, using the guidelines given for what the designers intend DC 10, 15, etc to mean. I love that! That's the sort of DM rulings that are perfect!
Note that this is different than DM rules interpretations. Rules interpretations come into play, not when the rules say, "the DM decides what happens at point X" but when the rules are unclear and the DM has to figure out how he wants to deal with them. Often DMs codify these in house rules.
DM rulings work well insofar as the system is set up to support and encourage them.
However, most aspects of 5e D&D are not set up like that. The rules themselves say "this is how it works," not, "at this point, the DM decides what happens based on considerations A, B, and C." This isn't rulings, this is interpretations.
Many of the rules of D&D are very clear, specific, and absolute. It doesn't say, "the DM chooses which ability score a player adds to an attack roll based on the way the player describes his character's attack," or "the DM might allow a character to make a saving throw (as chosen by the DM) to resist the effect of this spell..."
The game isn't designed to work like that--as a rulings based game. It is designed to function as a rules-based game with a few areas (setting DCs for ability checks is about the only one that comes to mind in the core system) that allow for rulings. Character creation and gameplay assumes a consistency of rules applications. With character classes designed around a set of features that interact with a core rules-based system, it is incoherent and bad design to leave rules unclear and/or contradictory and then just give permission to the DM to fix it as he chooses.
That's the problem here. The game is broken and they are saying, "ah...let the DM fix it."
Why is this a problem?
As a DM it is a problem because I feel I am doing a disservice to my players if I don't make it clear how I interpret vague rules from the beginning of the campaign (in the same way as I feel it would be unclear if I applied a bunch of house rules without telling them upfront). As a player, I would have trouble making a character if I didn't know how their class features were going to work until they happened to come up in play. Just imagine I'm making a character whose concept involves a lot of sneakiness both inside and outside of combat, based on my interpretation of how Stealth works. If my DM interprets stealth rules significantly different from how I do, I might end up not being able to do most of what I intended for my character. Why don't I just ask the DM before the campaign, you ask? Well, I probably would, but what about all the other rules he or she has to create their own interpretations for as the rules progress that neither of us might think of until they come up?
I hate putting my players into such a potential situation, so I have to write up my own clarifications for them before the campaign even starts. It just isn't fair that a player can memorize the PHB and still find out things work differently than they expect once they are 3 months into a campaign and have it damage their character concept.
The basic point I'm trying to get at is that going into a game of D&D with vague and ambiguous rules (as opposed to the perfectly acceptable areas where rulings are an explicit part of the rules) is a recipe for conflict in rules interpretations.
And this is coming from a DM who leans towards liberal interpretations in favor of his players, and always accepts rulings from DMs with whom he plays.
