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In short matches, my feeling is that the main factors underlying success are:
1. bidding games aggressively
2. good declarer skill
3. good defence
Games arise often, and afford chances to win or lose a lot of imps.
Declarer skill matters on all boards, other than passouts, as does defence.
Slams.. not so much. Easy slams are often pushed while difficult to bid or difficult to play slams arise too infrequently to be a big reason for winning or losing a short match.. they are important when they happen, but you may go several sessions without one.
Partscore battles are not very important in short matches, in my experience. Oh, every now and then you have a match in which there are no game swings or big blunders, and the partscores matter, but even then, my experience is that most swings happen due to play or defence, not outbidding the opps.
Mistakes matter, but most of the mistakes are included in the bidding of games and the play/defence. There are occasional outright blunders, but these usually have the effect of turning a moderate win into a blitz.
As the match gets longer, the weight accorded to these factors shifts. I think a large part of it is that most of us only get to play long matches against players of at least approximately equal skill level.
Personally, in local regionals, in a KO, I always end up in Bracket 1, where almost all players are good enough that they bid games aggressively, and have basic competency in declarer and some ability on defence. And in the later rounds, the competition gets even tougher... to the point that we can be playing teams with members significantly better than we are.... Passell/Wold was the last experience I had.
In those circumstances, partscores become more important, as do difficult to bid or play slams... because the teams are more evenly matched in the easier areas.
Luck also begins to play a role, altho conventional wisdom has it that luck usually evens out. I accept that, but 'usually' is not 'always', and two evenly matched teams may well battle to within a game swing, and find that the opening notrump range, as a frequent example, either caused a swing in the bidding or a swing based on which hand was on lead.
I also agree that mistakes decide matches more than does brilliance.. but there are two types of mistakes. There is the unforced error, whether through lack of skill, loss of focus, or a 'forget'. then there is the forced error, in which the opps place you in a difficult position and you take the wrong view. (BTW, especially in the unforced error department, luck can really play a role. Sometimes you make a bonehead bid or play and the cards just happen to save you...other times, a minor slip costs you 17 imps. I have been the beneficiary of the first and the victim of the second more than once).
I think this latter category, of forced errors, is most prevalent amongst strong teams.. with aggressive and brave bidders. I have marvelled at some of the top pairs for their ability to handle these situations... I think that this is a large part of what separates the upper echelon (say the top 40 - 60 players in the world) from the next level... and I say this knowing that justin, for one, thinks that there is a significant difference even within the top 50. The top players both create more of these situations for their opps than do the rest of us, but they also go right, when confronted with them, more then we do.