Yashin introduced a lot of the goalkeeping techniques you see today. Also a brilliant shot-stopper in a time when the ball was basically a rock. His agility was superb and he also saved a lot of penalties. But keepers were quite small then - Yashin himself was just over 6' tall. Our great goalkeeper Combi from the 30s was only 5'8". Planička, another great, the same. Up until the 60s it was common for keepers to be up to about 6'1", but the game was based a lot on shot-stopping and agility.
If you can innovate a position then it's difficult for anyone who follows to equally claim a position as the greatest.
It's interesting to see how being an ice hockey goaltender would impact on his agility and reaction times, as well as techniques and style. Much the same way that Peter Schmeichel incorporated handball techniques into his goalkeeping, like his 'spread eagle' charge against onrushing attackers.
Buffon was not an innovative keeper, but he has always had perfect fundamentals in pretty much everything, and a big personality to go with it. I think this and levels of concentration are equally what make a great goalkeeper. His penalty saving might only be a 7/10 but that's an art form in its own right. Average keepers can be expert penalty savers.
When you look at modern goalkeeping you'd suggest that Buffon's passing from tight areas but not be up to scratch for an elite keeper, but he's adjusted pretty well. He was an outfield player up until fairly late in his youth, but you can't re-train a style at 35+. I always remember in the early 90s when the back-pass rule was introduced, a few big keepers struggled because suddenly they had to kick a moving ball under pressure.
Now goalkeeping has become about sweeping, kicking and shot stopping. Less teams play authentic wingers with crosses coming towards the 6 yard box, so there are a few more keepers being 6'1". Plus everyone deals with the high ball by punching, such is the advancement of the ball technology. But the keepers are still 6'4"+ in general, as the perfect keeper would have that aerial ability and reach as well as being able to kick and have great agility.