From time to time I get feedback that I focus too much on save percentage. The main criticism is usually that it is folly to rely on only one stat, and that a well-rounded analysis should take all the available numbers into account (e.g. GAA, wins, shutouts, games played, etc.).
The one stat criticism sounds pretty reasonable. It seems intuitive that adding more information is a good thing. It is also undeniably true that save percentage does not tell us everything. It is important to know how many shots the goalie faced to be able to assess the level of randomness in his results. We should also take into account how many of those shots came on the penalty kill, and whether there is evidence of any other team factors like shot quality effects. It also seems clear that goalies have some small impact on shots against, and that should be taken into account.
What is usually suggested in place of a save percentage analysis is to take all the different stats, apply a weighting to each of them, and then calculate the final rankings based on all the different inputs. The problem with this approach is that it simply doesn't deliver on its promise. It does not actually measure a lot of different things that a goalie does. What it does is count save percentage 4 or 5 times, and then add in a bunch of irrelevant team factors. This is because every commonly available goalie stat is essentially a different way of restating save percentage and shots against.
Let's go down the list. GAA, as I've pointed out many times, is equal to (1 - save percentage) x shots against per game. It's handy as a shortcut to compare goalies on the same team, but if you're looking at rivals it's much better to try to make sense of the two underlying components than to try to assess their combination.
Shutouts are very obviously driven by a combination of save percentage and shots against, only we are adding in an arbitrary cutoff (why is 0 goals against worth so much more than 1 goal against when teams win 90% of the time when they allow a single goal?) and a healthy dose of small number randomness (shutouts are infrequent, which means luck is often the difference between something like an ordinary 5 shutout season and a great 8 shutout season).
Wins are determined by goals for and goals against. The goalie has almost nothing at all to do with goals for, and goals against are determined by the same two familiar factors of save percentage and shots against.
Goalie's World magazine has a fairly typical "computer ranking" that puts a weighting on each of the traditional goalie stats. On the surface, it looks like it is measuring a lot of different things. However, break down all the stats into their components and it is essentially only tracking three things: Save percentage, shots against per game, and how good the rest of the team is at scoring goals.
Given that only one of those three variables is mainly determined by the goalie, it makes a lot of sense to me to focus on save percentage. That doesn't mean raw save percentage is a perfect stat, because it is not. Step #2 is to make any necessary corrections (e.g. shot quality, special teams adjustments, shot bias) or try to incorporate any other results (e.g. shot prevention). What does the goalie do to win games other than stop pucks? If we know what those things are, then we should focus on them directly.
Maybe a goalie helps his team with his puckhandling and prevents a shot against per game. Preventing one shot is the same as preventing about 0.1 goals, which based on a typical game with around 6 total goals is about 1.7% of the total team contribution. How are we going to see that small effect show up in the overall win total? We might not see it all through the noise of how good the team is at territorial play, how good they are at shooting, how disciplined they are at taking penalties, how good they are at faceoffs, how good their coach is at matching lines, injury luck, shootout performance, etc. However, if I increase the goalie's shots against totals by the estimated amount of shots he prevented, he will get credit for them and that will boost his save percentage.
One other commonly cited justification for relying on wins is that certain goalies help their teams win by performing better in important situations. The problem is that most of the time we can predict quite accurately what the standings will be based on goal differential. According to Alan Ryder, goal differential explains 94% of wins. This suggests that any "clutch win" effect is likely to be either small or influenced by luck.
Maybe over the course of a season a team wins 2 more games than expected based on their goals scored and allowed. Even in the unlikely event that this is entirely because of the clutch skill of the starting goaltender, that's going to be a net result in a similar range as shot prevention (about 0.1 goals per game, based on the standard goal differential that is usually required to produce 2 extra wins). This again would make up a percentage point or two of the total team effort, and just like the shot prevention effect is likely to be hidden from sight among the hundreds of player interactions, coaching decisions and random bounces that determine the outcome of a hockey game.
For goalies that we suspect have an additional effect on team play, I think we are better off determining the effect and using that to adjust the save percentage numbers. It is of course tricky to estimate things like shot prevention, but it is much more tricky and uncertain to try to somehow estimate a goalie's shot prevention effect directly from his GAA. Or to assess the contribution of a goalie based on his career wins number. Both of those are complete guesswork, and far worse alternatives than some of the methods already proposed for dealing with some of the unknowns (e.g. comparing to backup goalies, or estimating the effect by combining our subjective evaluation with our knowledge of general performance ranges for a particular skill).
I focus heavily on save percentage not because save percentage is perfect and tells us absolutely everything, but because I think the other stats just don't bring an awful lot to the table. They are merely different ways of telling you how good a goalie was at stopping the puck and how good his team was at preventing shots.