I know that Avatar has more in common with Aliens than it does with Titanic, but there are three notable similarities between Cameron's last two blockbusters, aside from the one piece of music that always sets me to thinking that the ship's about to hit an iceberg (it's in "Scorched earth" for anyone who has the soundtrack).
(i) the defining image. Both films have one image, above all for me, that encapsulates the unfolding disaster and that will forever remain in my memory. Both images do not include any of the main characters from the film, but are absolutely striking and absolutely beautiful, yet convey a real sense of horror. In Titanic, it is the image of a drowned girl or woman, splayed out underwater, her dress moving gently about her. In Avatar, it is the image of a pa'li (or direhorse if you prefer the English) on fire and galloping through the burning forest. Neytiri is observing this, but she's not in the frame. This is a nightmare image, yet slow-mo turns it into one of strange and compelling beauty. Beautiful and terrible at once.
(ii) in both movies, one of the main characters (two in Avatar) is (are) bound and trapped as the disaster unfolds all around. In both cases, neither really deserves to be there. Jack is handcuffed below decks for stealing a necklace, when in fact he has been framed. Jake is tied up -- for what exactly? I think it has more to do with Eytucan's being angry that Jake has distressed his daughter, and a general anger at the skypeople than anything Jake has done personally. Unless it is just a case of wanting to shoot the messenger? Or because he has withheld information that he wouldn't have been allowed to deliver and wouldn't have been believed? He has, in fact, betrayed them through his reports, but they don't know that. (And for him it wasn't a deliberate betrayal. I imagine he had forgotten, if he were ever aware, that Quaritch was looking at these as well has his official reports, which had become a lot more guarded.) In both movies, someone has to rescue the main character rather than their being able to get themselves out of trouble.
(iii) something big falls! Yeah, this is kind of obvious, I know, but it still leapt out at me -- hometree almost trembling in the air, as did Titanic, then canting sideways and going down -- both achieving the same kind of "splash". Maybe it's just the cinematography -- not sure -- maybe it's just that it is the same type of event!
Post script: For the record I'm an Alien (Ridley Scott) rather than Aliens (James Cameron) girl -- much prefer the suspense than the action-shoot-'em-up kinda thing in those two, and yet here I am absolutely loving Avatar . . .