WyzardWhately said:
I get pretty tired of GMs complaining that this spell or that spell "ruins their plot." You know what the solution to that is? Don't have a plot. Have compelling NPCs and organizations and whatnot in the world who are trying to do varying things. Trap the PCs in the middle and let them make their own decisions, and find their own solutions. If you need the characters to follow your plot, or else everything is going to be "ruined," then go write a novel.
I cannot count how many GMs I've seen try to bring the hammer down on the PCs at every turn to preserve some half-assed "story" that wouldn't have held up in a ThunderCats episode.
As a world-builder that my players later can use as a playground, I still have to take all these abilities into account. And that can create some very bizarre worlds. That probably won't really change in 4E.
And then, I am not such a great DM that I can give my players total freedom. They can't go everywhere in the world and expect me to have that part ready for them to interact with. I am happy if I manage to keep my adventure idea in mind and don't forget important details, or details I added for fluff/flavor/enjoyment.
Just as an example for something that is game-breaking (if you didn't take it into account) in my view:
Let's assume I add a murder scene happens. At 1st level, if the players choose to interact, they will have a lot of leg work to do. (But Detect Evil can totally wreck the idea of a good-pretending evil guy)
At 5th level, they have spells at their disposal that willl allow them to compel others to tell the truth. That changes how the adventure must be built. The culprit needs to either have counter-magic, or not get even close to the characters. At higher levels, I have to take into account Speak with Dead (which is a pretty nice spell for this, I admit. It's not so hard to make it useful without game-breaking). As levels grow, the number of spell that become "auto-win" buttons increase if you don't take them into account. The problem is predicting when this will become true.
Sure, I don't have to introduce this kinds of scenarios at all. But the point is - it could have been fun doing so, but if you know a spell can shortcut everything, it's becoming a lot harder.
But more importantly, this doesn't have to be a "fixed" plot. My only assumption is that the PCs will be motivated enough to find out that who was the murderer. How they do it, I don't care, as long as it can entertain us for a session.
With guidelines for what to expect in tiers,
1) I know what I have to take into account when coming up with adventure ideas and scenarios. I don't have to care about Scry-Buff-Teleport at Heroic Tier, and I can make it nearly a requirement for Paragon level.
2) I have a lot of time to use the concepts that fit in there. This is probably the most important part, since spells always came with a level limit. But they came early and often, and that made it hard to have a free-form campaign over multiple levels.