But in 5th edition we aren't talking about something that is outside the rules, that is all a part of the rules.
What is part of the rules, changing a spell? What are the guidelines for that?
I'm not talking about changing Necrotic to Fire types of changes. Straight up changing a power can have serious implications in 4th edition.
Like I said before, no more serious or self correcting than changing a spell in 5e. This argument of balance is an additional red-herring. Yes, 4e is more balanced, as a whole, than most other forms of D&D. But there are still plenty of things that can be unbalanced/broken in it, but it still remains quite balanced as a whole. The notion that changing a power has such serious implications is, for lack of a better word, ludicrous.
All you need in 5th edition is Imagination + Ability Scores to do what you want to do. That is the way the game was designed. If I want to flip over a rail, grab a chandelier and come down on an enemy with my sword; all I need is to roll a Dex check first followed by my attack roll.
This is certainly the red-herring of the conversation because the truth is that it all totally depends on the DM. It's not like 5e all of a sudden introduced something that didn't exist before in RPG's. Nothing in 4e prevented you from using Imagination + Something Else either. In essence, the base of the systems is the same.
In both systems this is an ad-hoc ruling, but 5e gives the DM precious little to guide how to consistently make that ruling. In addition, in 5e it is always a DM ruling with variable certainty. What is the DC of vaulting over the rail, if the DC doesn't already exist in a table somewhere how do I determine it consistently, how about DC of grabbing the chandelier, does the PC take damage from landing on the enemy, does the PC need to make 4 rolls or simply one (flip, jump, grab, attack)? If DM A decides to use Dex Check (flip rail, grab, jump, land) + Attack Roll to determine the outcome, but DM B decides to use DEX Check (flip rail) + DEX Check (grab) + STR Check (jump) + Dex Check (land) + Attack roll for the same the odds of accomplishing the task are possible in one instance and almost impossible on the other instance.
In 4e the variable certainty is reduced allowing both the DM and player to have a better idea of the real chances of success, because there are solid guidelines to make an ad-hoc ruling. For example, there are not 4 rolls to make but one (Acrobatics). What 4e also provides to the players are concise pieces that don't require ad-hoc ruling. If the player had a power that allowed him to do a "Death from Above" maneuver the DM can simply use that as the basis for the action. And if the "Death from Above" maneuver doesn't exist for the class but it does for another class the DM can use that as a basis also. One system gives the DM robust tools for adjudication and extrapolation, the other kind of gives him the idea that he can make these ruling but little in the way of tools.
4th edition has a hard coded way of handling things because it was specifically designed with the grid in mind while 5th edition was not. Theatre of the Mind gives you a lot more freedom than grid based.
4e had effects based powers to give players certainty in the execution of their actions. That has little to do with the grid. After all a 5e fireball does Xd6 damage consistently and the DM doesn't get to arbitrarily change that. That also has nothing to do with a grid. DM rulings are great, but as a DM and a player I prefer when my rulings can actually be consistent across the board, and also across game tables. I DM and play with several groups. If I play with group A today and group B tomorrow, as much as possible, I'd like my character to be able to do similar things without DM fiat. The discussions on these boards about, "My DM is screwing my warlock" are the kind of thing that dishearten me about 5e and its "rulings not rules" mantra.