I think that Ron Edwards is right when he says that system matters, but I think he overstates his case. The impact of system on game-play really depends on the level to which game play is reflective of system. And I actually think system/rules is only one mediating factor in the social construction of the game as activity. The game, the social dynamics of the group, the pre-existing expectations the players bring to the game, and the ways that players and GM play out their roles, for instance, all have an influence. Each session of a game exists in a social and historical context defined by all members of a group, and each member has an individual experience even as the group has a collective experience.
So in short, system matters, but so do a lot of other things.
I have been thinking about this a lot as we play 4e in my friend's campaign. I'm having a good time, and while I am enjoying playing with the rules, I imagine I would have the same kind of fun playing just about anything, seeing how to play with the rules of the system while engaging in some interesting role-play moments. However, the structure of the sessions is clearly being influenced by the rule system. First, it clearly does a lot to structure the kinds of characters at the table. The kinds of character's at my friend's table aren't the kinds I would expect to see if the same players were in an Athanor game. Second, it structures the kinds of things that happen in combat. But interestingly to me, it structures the pace and the structure of the game.
To be honest, my issue with 4e isn't unique to 4e. It's a lot like what I experienced with 3.x, Hero System, and GURPS, for instance. But what is striking me about 4e here is a separation of tactical and roleplaying. There is a tactical portion of the session, where we do battle and run the game strategically to overcome foes, and there is a roleplay session where we attend to the plot. It doesn't come in a particular order, but the split is palpable.
Now, I don't necessarily think this is a problem. I have happily played Hero System this way for years, and did so with increasing unhappiness with 3.x, but as I have gotten older, the systems tweaking and tactical play elements of RPGs are less and less attractive. This is part of the reason I declined playing in a friend's playtest of Pathfinder, and part of the reason I am interested in games like Swords and Wizardry (and have raided Tunnels and Trolls for some of my ideas, too.)
Frankly, part of the hobby has always craved some sort of complexity as part of the development of the game system. The added mechanics of games ranging from AD&D to Runequest to Chivalry and Sorcery, Aftermath,, Morrow Project, Rolemaster, and a whole slew of even more complex games has been driven by a desire by some players to have more rules covering more situations with more possible results. I tend to think of it as a sort of game mechanics porn that some people are just compelled to experiment with.
But to me, that stuff is less and less exciting. The last time I had a great time GMing a game was when I was running Feng Shui, when I felt happy throwing out world canon and with playing fast and loose with the rules. That carefree attitude toward Official Setting and The Rules is the hallmark of games I like to run.
So I need to keep thinking about where I can avoid having set rules in Athanor, and leave more room for making stuff up. This is why I'm pleased that my rules document is 8 digest pages, and that the rest fits into a short stack of very thin booklets. I want to keep it that way.