It could still be terrible. I'm not denying that.
And, yeah, Ritchie doesn't exactly instill me with confidence.
(I certainly don't hate him.)
... but the knee jerk reaction seems horribly incredulous. The trailer didn't look awful, and it certainly did not give any indication that the film will be awful. It looked entertaining. As for Ritchie being unable to handle such a complex character, I say leave that to Downey and the screenwriters. He isn't writing the film.
It could definitely go either way.
The problem, from what I can see (on Internet cesspools like AICN, CHUD, etc.), is that people have this unabashed hatred for Guy Ritchie in general (which I don't understand) and not for his weaknesses as a director. That, and an aversion to Sherlock Holmes using martial arts on screen, which is something he's always been skilled in.