I just wanted to add a quick link to/support of this article over on Gnome Stew, where the virtues of, and reasons for, saying no are nicely spelled out. Damn skippy! For too long, the Viking Hat has lingered in the closet, and, as this thread on RPG.net shows, there are way too many players out there who think that anything they can come up with ought to be allowed, as the GM (and other players) only exist to serve their needs.
I use the Viking Hat icon in Earth Delta for a reason. The more freedom a game allows, the more options it presents, the less able rules are to cover all edge cases. Wizards Of The Coast has a clear economic reason to push “say yes” — it sells more books if the players think they can use anything in them in their campaign. I also understand why they didn’t want to publish a huge host of detailed modifiers to every rule and power when one of their goals was to get away from over-crunchiness. Ironically, one reason sometimes given for this was that “too many rules remove the DM from the equation” (I disagree; within the limits of playability, detailed rules give the DM the best framework for making judgment calls on those cases the rules don’t cover), but, even if that was their goal, they then made sure the DM didn’t re-enter the equation when they advocated a policy of “The rules always work”. By this I mean, they explicitly stated the DM shouldn’t try to overrule a situation where a power/feat/magic item/whatever didn’t seem as if it could function; it should function and the DM should find some way to rationalize it. Knocking gelatinous cubes prone is one classic example; another is the fighter power Come And Get It, which allows a fighter to Pull nearby targets to him and then whack them. It’s a great power, in terms of effectiveness, but it often makes no sense. It doesn’t attack Will or have the Charm keyword, so it isn’t any kind of psychological assault; it works, as does all forced movement, even on Immobilized creatures — so a monster that can’t move when it wants to can somehow move up to 10 feet closer to the fighter to get whacked on. However, a policy of DM judgment calls on commonly-used powers would make some powers far more prone to “No, that doesn’t work” than others, and so the official rules stance has to be “It just works, dammit! Now don’t bother us, we’re rolling on the Quarterly Market Strategy Change table!”