When did you watch it? When it came out, it was technically impressive for the computer-generated graphics, which included a lot of highly-detailed and expansive "organic" stuff like forests.

Here's some quotes from the Roger Ebert review from the time:

https://www.rogerebert.com/reviews/avatar-2009

Like “Star Wars” and “LOTR,” “Avatar” employs a new generation of special effects. Cameron said it would, and many doubted him. It does. Pandora is very largely CGI. The Na’vi are embodied through motion capture techniques, convincingly. They look like specific, persuasive individuals, yet sidestep the eerie Uncanny Valley effect. And Cameron and his artists succeed at the difficult challenge of making Neytiri a blue-skinned giantess with golden eyes and a long, supple tail, and yet–I’ll be damned. Sexy.

Cameron promised he’d unveil the next generation of 3-D in “Avatar.” I’m a notorious skeptic about this process, a needless distraction from the perfect realism of movies in 2-D. Cameron’s iteration is the best I’ve seen — and more importantly, one of the most carefully-employed. The film never uses 3-D simply because it has it, and doesn’t promiscuously violate the fourth wall.

I mean, I remember being underwhelmed after I went to watch Avatar with a friend who was deeply impressed, but it did show off a lot of render capability for the time. I'd call it more impressive as a tech demo.