Actually, 6teen was produced 4 years ago. Even now, it still holds up! The animation was done with an impressive Flash-like animation software called ToonBoom Harmony (and also with a more advanced version, ToonBoom Digital Pro). I fell in love with the animation & designs when I first saw it on Nickelodeon.
If you like the animation in 6teen, you should check out Grossology (which began its 2nd Season in Canada on YTV, and Season 1 is still running on Discovery Kids in the US), which has the same exact character designs & animation! It's one of my favorite shows.
I take it you're not familiar with Nelvana (which has been around since the 70s). They've been very diverse. 6teen and Grossology are the only two shows by the same team (thus far).
And you have to remember, this was the same studio that gave us Rock & Rule and the Beetlejuice cartoon, for example. Not to mention Clone High USA!
Both are awesome! You should be able to find Rock & Rule in the US. Look especially for the 2-Disc Special Edition!
Clone High USA came on MTV for a while, and then got cancelled before the last five episodes could run! (But then, that's MTV for you; They seemingly lost interest in animation after the end of Daria.) Of course, the show, in its entirety, aired in Canada (on Teletoon's "Detour" program, Canada's version of Adult Swim), and the entire series is on DVD there! Otherwise, try a YouTube search for clips there. It's a pretty fun show! It's what TDI kinda' reminded me of.