This is true to a degree and like it or not, Ledgers sudden death did a lot to inadvertently market that movie, however before that trailer hit a LOT of people (and I'm talking real mainstream here, not GAFers/people that frequent film sites or message boards) didn't even know Ledger was playing the Joker, or they knew and just didn't care.
That trailer came out in December a good month before his death, and I remember that once it hit and word of mouth spread, I was over hearing people in bars and other public places talking about how awesome this Dark Knight movie looked. Also remember that Begins did well at the box office, but not exceptionally well. I know people that hadn't even seen Begins, nor really wanted to, that suddenly got excited for TDK because of this awesome trailer that they'd just seem with I Am Legend.
As for the Avatar trailer I can understand why you personally don't want Jim to show too much. I can appreciate that you want to go to a lovely 3D IMAX on opening day and be wowed with fresh eyes, BUT you need to think about the mainstream here.
A short teaser that shows Camerons name, a quick cut of a Na'vi, and then the Avatar logo is not going to cut it. It's too late for that now That is not going to captivate the mainstream, who know next to nothing about this projects existence, and they will be left scratching their heads.
If they had been steadily marketing this movie for the last year then sure a trailer like that may suffice, but that is not the situation we are in.
Cameron needs to WOW with the trailer to get people to pay attention otherwise the movie is gonna bomb at the box office.