The Last Airbender series was stronger than Korra all around, IMHO. The episodes that were episodic in nature were still stronger than a lot of what happened throughout Season 2 of Korra's overarching plot. I strongly recommend that you DO watch it, much of what you would take for granted in Korra is stuff that is just being discovered in the Last Airbender series.
For example, Toph Beifong (Lin's mom) is the first person to ever discover Metalbending, which is something that is seen regularly from the very beginning of Korra. Same goes with that Echolocation thing that Lin does in Season 1, this was first used by Toph Beifong in Last Airbender, though I don't know if she's the first to ever use it. Lightning-bending is a highly disciplined version of fire-bending in Last Airbender, whereas it's pretty commonplace in Korra. Same with Bloodbending - if I remember correctly, this is discovered by the waterbender chick from Last Airbender (the one that ends up marrying Aang and is Tenzin's mom).
Plus there's the whole history of the Last Airbender thing that you just don't know about, so the fact that there are suddenly a whole bunch of new airbenders wouldn't have quite the same impact on you as it would on someone who's seen Last Airbender. There's just so much history that you're missing because you haven't seen Last Airbender that would totally help enrich your whole Avatar world. So yes, as I said, I highly recommend it!