I don't think it started sucking per se. It just COMPLETELY changed focus and direction after the first few question arcs.
Those first arcs were scary as fuck, everything seemed random, etc. etc.
In Twin Peaks, btw, there was a long running debate as to whether there should be an "answer" to the questions posed. Of course, the studio execs stepped in and made the writers have everything worked out in the end.
Maybe Higurashi isn't that different. It was way more awesome when you didn't know what was going on or what to expect. The second season is like a watered-down Moetan.