It means that CBS' Sherlock/Watson have been through more stories, more episodes, and more interactions and time on screen, yet their characters are still interesting to watch and have the element of feeling like there's more to them. Sherlock operates in 3 episode blocks of great length. They have long episodes, but a lower exposure to interactions and stories. Whatever spark to their characters are there, its somehow already tired out with less exposure to the audience.
If you don't understand audience exposure: Your TV characters, even the most cliched, have a freshness to them at first and a potential to interest. Does TV Mom #345678 have some secret talent? Where will her story go? Over time, characters get stuck into types. Sometimes they dry up in those 'types'. There isn't much more to them. Sometimes the writers address this by changing the character's position within the story or basically hitting the reset button on them. A character's potential as a character has limited room. Some shows do more with this than others. The more exposure, the more episodes, the more they repeat behaviors, the more they become exposed to the audience and the more the critical audience will grow weary or disinterested in them. (Though it would seem the non-critical audience loves the predictable character. And the Community fan thinks the predictable character is cute TV and pointing giant fingers at it is clever. )
In BBC's Sherlock's case, we have Sherlock as the Psychopathic Autism Superhero. This is a pretty old and standard take on Sherlock, so what else is there? He's kinky? That kind of falls in line with standard character. So is it bad that he's a modernized, tumblr-sexy Sherlock? Not so much. However, it means his interest potential has dried up pretty quickly. And Watson won't ever truly challenge him, in holding to the prototypical Watson character fashion.
