Why is this considered good?

Rating: 4/10 (11 October 2008)

I've never understood why this film garnered such critical and financial success. I've heard it said that it made so much money because teen girls kept going to see it over and over again due to Leo DiCaprio, but I don't think that's the case; I think some people just really liked this film.
Which of course mystifies me. The plot is the stockiest of stock plots; high-society girl falls in love with lower-class, but fun-loving rogue. Cue sappy music. But oh, conflict! She's already engaged to someone else, but he's kind of a jerk and she doesn't really love him (I know, it surprised me, too). At the end, the boat sinks, of course, and Leo DiCaprio dies, apparently having realized that the movie he's in is no good.
Do you see the problem here? Every aspect of this film is utterly predictable, every single character is a character we've seen countless time in other, better films, and yet the stupid thing is almost three-and-a-half hours long. It spends so much time "developing" these characters and situations that we've all seen before, one has to wonder if writer-director Cameron actually thinks he's done anything original here. Indeed, I found myself predicting scenes before they happened, and I was almost always right on the money - with the exception of the fact that the dialogue I had in my head was less cheesy and hackneyed than what was displayed on screen. The only mildly interesting part of the entire affair is the sinking of the ship, and even then I want everyone to hurry up and die so the movie can just END already.
Unfortunately, the actors aren't even at their best here. DiCaprio is downright dreadful, which wouldn't be so unfortunate if he hadn't displayed far greater acting talent than he did here in just about every other movie he's ever been in. Winslet doesn't fare much better, but she does what she can with what's given her. What's worse, great actors and actresses are severely underused, as in the case of Kathy Bates, who is given a criminally small amount of screen time.
In short - uninteresting characters, a predictable plot, cringe-inducing writing, and decent-to-dreadful acting make viewing this film a rather unpleasant business. The ending is good, but that's only by virtue of the phenomenal special effects - and special effects do not a good movie make (see: the Star Wars prequels).
So, with all that in mind, again I ask ... why is this considered good?