Quasqueton said:
If a DM has something happen, or an NPC do something that "breaks" the rules, does it bother you in any way?
If an NPC does something that's normally impossible (for someone with her abilities), she'd better have a magic item, template or similar explanation for it.
If that's not the case, i.e. the DM is just making stuff up, that might bother me, depending on the nature and scale of the incident.
Is it OK if the "something" is interesting, or makes for a good story?
If what you're planning is that good, you also should be able to provide a satisfactory start. Including a reason why some things temporarily don't work as they ought to.
Anyway, it needs to be interesting or good. Otherwise, why would you even go to this effort and confuse your players with inconsistent rulings?
If a DM prevents or disallows a PC from doing something within the rules, does it bother you in any way?
That's where I draw the line. If you cross it, there need to be some serious mitigating circumstances for me to play with you again. (Note: Playing isn't a big interest of mine - I'd rather GM. So if I am to do something I don't prefer, it'd better be worth my while. YMMV.)
Once again - there's a lot you can do with magic (items), templates, etc. Need a to give a character the (e.g., spell-like) ability to use
mass charm? Well, find (or create) a template, give it to the character, and find a reason why she has it. Same thing with items.
Sure, that doesn't make a railroading,
if applicable, much less of one. But if you're going to resort to heavy-handed tactics, the least you can do is find an in-game explanation. That doesn't break suspension of disbelief as much and also means the PCs might well have a chance to deal with this special power (or its source) later on.
Is it OK if the "something" would make the situation less interesting, or undermine the story?
No.
Is it good DMing to bend/break the game rules for the sake of a good story?
That depends on what your players want.
As a player, I don't care for stories that require heavy-handed DM intervention so they can follow a script or some such.
As a DM, I'm willing to gently nudge things a little every now and then if it makes for a better game
for my players. If such very subtle methods don't get the job done, I'm unlikely to intervene more forcefully.
Is it good DMing to make the story work within the rules?
Obviously, assuming you can indeed pull it off well. It's not always easy, after all.