They will almost never do the thing you want them to do
That has to be one of the truest things said about good RPers I've ever heard. The hardest part is learning as a DM to not punish your players for thinking for themselves. Once I spent a good solid week, planning, and creating all the material I'd need for a dungeon, it was a good 4 to 6 sessions of material. Upon reaching the entrance of the dungeon, what do my player decide? That they'd prefer to trudge through the jungle during a typhoon then enter into and I quote "What is probably a conviently placed gate way to hell", my players were paranoid. I later used the dungeon else where, and just tweaked it a bit, but it was painful to have them simply decide to pass it by after all the work I did.
I also love creative spell usage. Like the time the group was up against a pair of trolls but lacked the means to kill them. So the mage used his last spell, Grease, to cover them in well grease and then threw a torch upon them. I decided the oil was very flammable and so the trolls went up in a blaze of glory so to speak. When I asked him why he had grease (since its not a commonly used spell) he replied with "Because you never know if there's a door that'll be squeaky and need a good greasing up."
These at the end of the day are the kinds of moments I want to be able to reproduce in a game. A scripted game only so much, because you have to anticipate the players actions as much as possible, but can only give a couple of actual options. An open environment, however, with someone who can change, add and subtract things at will would be a fantastic platform for just that kind of RP. The neverwinternights series of games kind of accomplished this with its modding community and PW (aka Persistent World) servers.
I really do hope I can craft both types of experiences with Elemental, because it looks to possibly be an excellent platform for it.