Surely in the basic sense that the game can be played without one of the players but not without the GM? I also would say that the quality of the GM influences the quality of the game more than any particular player.
In those senses, yes. I took you to be saying something like the GM's pleasure in the game is the most important. If you weren't, I misunderstood. If you were, I still don't think I agree.
A good example of how early editions were more conducive to creative play, that isn't because of a simple gap in the rules, is the Command spell. Traditionally, Command lets the player choose any single-word command they want, with the DM working out the implications. The 4e Command spell lets the player choose whether their target falls prone or slides a few squares.
I think this is a good example. In 4e to get that sort of creativity the players need access to a dominate effect (ie the potential is still there, but not in the Command spell). Using the Command spell creatively in 4e is going to be primarily about exploiting terrain and positioning - I don't think that's nothing, but I do think it is not as open-ended as what dominate permits. (I've sometimes seen it suggested that fictional positioning isn't part of this stuff, but that's not true in my experience - eg using forced movement to impale a beholder on a stalactite (actual play example) is certainly exploiting fictional positioning.)
4e is an odd mixture of tightly-defined combat rules (tighter than classic D&D - Command is just one example) and loose, almost free-descriptor, non-combat rules (much looser than classic D&D in my view). If you are looking for non-tactical creativity in 4e play, I think that is where it is most likely to be found. But even in combat there are options for creativity that are interesting and (I think) distinctive, such as using Intimidate or similar abilities to deal psychic damage.
When a player comes up with some sort of plan that seems dubious based on my understanding of the fiction I try to assume that it makes sense based on theirs and view it in the most favorable light. I try to reward gumption in itself.
I think this is a good technique. I also think it could benefit from more emphasis in a lot of GMing books! 4e makes this easier than classic D&D, I think, because it gives the GM more support for measuring costs and benefits (eg p 42, more systematic set of conditions, etc) - whether that's a pro or con is probably a matter of taste.
On a different note, could you say more about how the 4e rules help you put pressure on the players?
I think I said upthread it has to be through encounter-framing and rationing extended rests (and thereby resource recovery, hence resource availability).
If the GM doesn't frame encounters that will tax the players' resources then there is fiction, but not really much mechanical play - at which point the fiction really needs to be very engaging, because it will be the only source of pressure. In combat, taxing resources means taxing surges, action points and daily powers; out of combat that means taxing surges, action points, daily powers, rituals, and also encounter powers if the non-combat situation will not permit a short rest. You do this by putting something the players (and typically their PCs) want on the "other side" of the situation, and then mechanically framing the situation in such a way that they can't get to the other side without using those resources (eg fighting, persuading someone of something who really has a different opinion, getting somewhere that's hard to get to, etc). This is partly why the reliable encounter-building tools are so helpful - they give you the guidance you need in doing things that, as GM, you
have to do (namely, frame encounters of known, typically high, levels of difficulty).
The rationing of rests can be done either via pure metagame/house rule - eg change the period from a day to a week, or do what 13th Age does and require 4 encounters before a rest - or can be snuck into the fiction, which is how I tend to do it: eg in the Underdark it is too stressful to rest unless you have a civilised roof over your head, and then ration the access to civilised roofs. You can then get feedback loops where the players spend resources on skill challenges to try and get access to rest points - I haven't formed a strong view on whether or not this is good for the game, but it is a feature of the game as I play it.
The WotC modules suck, in part, because too many of the encounters aren't hard enough (eg they are on-level, or close too), they have no real fiction driving them (so the players have no motivation to get their PCs to the "other side" of the situation except that the GM has the module open to that page), and there is no serious attention paid to how to ration resource recovery.
Comments/questions very welcome!