Having grown up with Saturday Night Live (at the risk of dating myself, virtually from the first season) and being a long-time fan, I've noticed that one of the weaknesses of sketch comedy is that skits tend to be high on concept and short on character. While many SNL characters certainly have broken out and achieved a certain cultural zeitgeist, rarely have they been strong or endearing enough to succeed outside the skit format. For every Wayne's World, there has been a Coneheads, A Night at the Roxbury, and Ladies Man.
Which is why 30 Rock--from writer/creator Tina Fey, former head writer of SNL--has been such a delightful surprise. Although the sitcom showcases Fey's comedy chops both as a writer and performer, she actually also has succeeded in developing a fairly strong cast of engaging and likable multi-dimensional characters that are engaging, surprising and unpredictably quirky.
But the depth of the rest of the cast shouldn't be overlooked. All of the characters are well defined and interesting, and Fay herself is endearing as Liz Lemon, the head writer of the show-within-a-show portrayed on 30 Rock. As Lemon, Fay shows herself to be a terrific actor and performer (her reactions and double-takes are priceless), who is willing to be the straight man as much as the comedienne. But she also has given the character its own quirks and personal issues, which adds to the character's charm. In fact, I would go so far as to say that with Liz Lemon, Fey has successfully carried on the mantle that began with the old Mary Tyler Moore Show by creating a character who is a strong, independent professional while still being vulnerable and feminine (or, in this case, geek-femininity) as well.
The bottom line, of course, is that 30 Rock is one of the funniest shows on TV right now. The strength of the characters, hopefully, will give the show some "legs" and longevity.