This is more a review of the presentation on modern 4k sets. I'm sure this was flat out amazing when it was released on blu-ray back in the day. Today, a good 4k set is more revealing. Does this film have any imperfections? To be honest, not really. Among FHD only releases available, Avatar is still tops. Compare this to the LOTR extended edition blu-rays. In some of those scenes, there is unsightly noise around certain objects. I thought it was slightly drizzling in a couple of scenes...nope, just noise. Avatar is leaps and bounds above this. Additionally, it really shows its prowess when the Eywa scenes come in. Spectacular contrast and vivid color enhance the definition. Still, when compared to the 4k HDR presentation of The Matrix, it's easy to see where Avatar falls short. Close-ups are not as crisp. Contrast throughout the film is a little weak. Because they did such a phenomenal job on the blu-ray for FHD displays, they probably saw little need to rescan and HDR grade the film. I bet, however, that if they did that the 4k HDR edition of Avatar would be exceptionally hard to beat.
The audio still rocks and competes with the best of them. If you have a stereo setup, make sure to put the film in English 2.0 mode. You'll be surprised at how much more alive and dimensional this is over the 5.1 track. I'm sure if you have a 5.1 setup, the 5.1 track will be awesome, but with a stereo setup flip back and forth between the two and you'll immediately hear (and feel) the difference.
Overall, great film and still very engaging even though it can't quite keep up with a more modern large screen 4k display (I'm viewing on a TCL 65R). Very much worth $10 or less that you can grab it for now.