I'm a system guy. I make no bones about it, though I'm a fair dab at settings too. Designing the StarCluster 3 system has been illuminating for me. Being an extremely abstracted framework system, it allows yoinking parts and pieces off and slapping new bits on to an unprecedented degree. In playtesting, I could - and did - swap resolution sub-systems in mid adventure, requiring only a change of dice and character sheets, pre-made of course, as I knew I would be doing this. This allowed me to look at the impact of various resolution systems on play as an isolated phenomenon, because all other parameters were identical.
This allowed me to prove empirically that system does indeed matter. The flavor of the games would change markedly depending on the resolution package I was using. The percentile StarPerc mechanic, for example, allowed a big random element in Quality, as chance and quality were entirely separate rolls. This meant skill was less important because it had no influence over quality of success. It also encouraged to a far greater degree abstract tactics. Players would freely take a penalty in one aspect to enhance another aspect, and there was greater control, with small gradations and three different aspects - four with the optional Active Defense rule - to tweak.
StarPool on the other hand, with its d20 dice pool with skill rank+1 dice rolling under stat and counting successes, has chance and quality tightly bound in one roll, both heavily influenced by skill. There are only two aspects - three with optional Active Defense - to manipulate, and the gradations are coarse. Consequently, players are less likely to use abstract tactics, and will tend to stick with their die rolls straight.
So, let us construct a sequence of events in StarPerc and see if it can happen in StarPool. We will take a tyro with a pistol, say skill+1, against a enemy threatening his family, friends, and shipmates, and he is the last defense. The character knows he's a tyro, so he will do whatever he can to improve his lot. Say he rolls a 25 on his initiative. He immediately changes this by 95 to 120, waiting and waiting to the last for the best shot possible, giving him a +95 to split between chance and quality. He bumps chance up by 25 from 50% to 75%, and gives himself +75 to his quality. He could also throw in traits or edges as appropriate. Let's say he uses his "patient" trait to give him an extra +10 to chance, now at 85%. He now has an 85% chance to hit, his damage is d%+75+20 from the pistol, so between 96 and 194 points, with an equal likelyhood of anything in between, and a mean of 146. That is a hell of a blow in one shot.
Now with StarPool, he has two dice to roll under his attribute. He can throw in his "Patient" trait for another die, to make three. He can knock his initiative back, giving him an extra die for each 3 points of initiative. Say he rolls a 5 - equivalent to the 25 rolled in StarPerc - so he can add 5 more dice by going up to 20. With eight dice, he is almost assured of at least one die rolling under his attribute of 9 for a hit, so lets assume a result of between one and 8 successes, most likely about 4-5. With each success worth 10 points to success, that is 10-80 points, most likely 40-50, +20 for the pistol. The range then is 30-100 damage, but strongly tending toward 60-70. Since Constitutions in StarPool are half that of StarPerc, if we double the damage, we can compare the two directly - low 60, high 200, very strong mean of 120-140.
"So what's the difference, clash?" you ask. "Looks like you pretty well nailed it, with high, low, and mean right close in there." The difference lies in two things, the strong propensity for the mean with multiple dice results, and the psychological aversion to taking advantage of the point trading/abstract tactics inherent in a lower variable system like StarPool. Players don't think statistically when they are in-character. Players think viscerally, and the transparency of the percentile manipulations, which can easily be handled viscerally, at the character level, trumps the obscure 3-points-of-initiative-for-an-extra-die of the pool, which can't. Players won't take that kind of risk, especially knowing they will most likely end up with the mean or near to it.
So, upshot is that the character will not hold out til last and give one huge bang with StarPool. I have never seen a player change more than two three-for-a-die units in StarPool. Never. In most combats, only one unit is changed, by one PC. That's usually it. Three-for-one, which is perfectly sound statistically, is not such an easy choice to make psychologically. Play changes to accomodate. Player Characters go with skills they are good at, and ignore skills they aren't good at.
So - at the group level, system is important. Yet play is not radically changed - it still follows fairly closely to the patterns established using the other system. It's just edge conditions that are different. Thus other things are *more* important in play - trust between players and between players and GM, internal logic of the game so far, estimations of the goals and resources of the opposition, and the inherent parameters of the setting are all *more* important to what happens in game, and they are extremely hard to quantify for the GM, and impossible for the designer.
So, does system matter? It very certainly does. Is it the most important thing? No, not by a long shot, not at the table. For designers, it's a parameter they can control, so it is far more important for them, but for the Group? Not so much.