I edited that 2nd trailer into one of my AC4 posts, earlier. I had not seen the 1st one, which is even better. I'm getting pumped for this.
This thread has links to those trailers without the dumb full-width link banners at the bottom. I downloaded one of those above, thinking that the banner was an overlay, but no--it's in the video itself. There's a fair amount of discussion about the game in that thread as well.
Edit:
Some first impressions. It all looks good, from where I sit.
