Contract extension: in contrast with the alleged 'slave contracts' some K-pop fans love to discuss, they have the highest artist retention of all the K-pop companies, absolute and percentage wise with even lots of 1st gen/2nd gen artists still being affiliated with SM after 10-15+ years. Probably also because SM seems to really help out their artists with their solo music endeavours and other individual activities right from the start until long after their peak.
Sticking with their artists: although SM suck big time are really weak, unresponsive and weirdly ineffective at slander rumor handling and reputation protection (their own and their artists'), they do stick with their artists and don't drop them in a blink at the merest hint of a serious scandal, like some others do.
Coaching/training: their coaching and training program is pretty good, especially their vocal coaching, even representatives and artists of other companies were impressed by SM vocal experts after they got a few vocal sessions. But also their dancing training, NCT's and other SM groups' dance skills are pretty good, Taeyong is a good example but not the only example of solid dance training.
... they used to include overall entertainment skills and training, which led to groups like Super Junior and SNSD being top variety kings and queens but also other SM groups and artists being good at variety entertainment - but unfortunately this doesn't seem to be part of the program anymore, probably due to the k-pop scene in general focusing internationally and moving away from (or prioritising less) Korean public and Korean TV and variety shows.
Not afraid to experiment: of all the big K-pop companies, ever since 2nd gen SM have been the most daring in breaking away from the mold and mainstream trend, not only in music but also in other ways. Going for large groups as 1 of the first in K-pop, having a tomboy like Amber or a fat guy like Shindong in their groups, having split groups (EXO-K, EXO-M) or rotational subgroups (NCT), SM Station, SM Classics and so on.