I scanned this forum back to 2012 and didn't see an official thread for The Last Airbender, though I did see some mentions of it.

The series was added to Netflix recently, so a lot of new people are being introduced to it (myself included).

I'm not really into anime, so I'm not sure if anime fans really consider ATLA to be part of this genre. Is it more like a kindergarten version of anime? Is it a "gateway series" that the non-anime fans into being more open to cartoons that are anime styled?

atla is a cartoon not an anime

Depends on, if you're japanese you would call it an anime. If you're not japanese then generally you would call it a cartoon since it's origin isnt japan

I wouldn't call it an anime, but I also think it's up to each and everyone what they call it. I haven't watched much of the show myself, it's really not for me.
What I have seen though is people absolutely lose it over the discussion if ATLA is anime or not. It's like watching kids argue over what power ranger is the coolest.

So tread carefully

Anime is simply the japanese slang for animation, if you show a Japanese person ATLA , they'll just refer to it as anime anyway.