Another vague comment. Are you saying that their system does not pay you your fair share? If other pools are not score based, what are they based on?
Most pools are based off proven unhoppable methods (DGM, PPLNS, PPS, CPPSRB). Slush's payment method is hoppable, though not as reliably as the old proportional methods. I don't think that's what most the responses refer to though. I think the biggest thing people have against slush's pool these days is the complete absence of Slush, especially since the little quirk in his round calculations still screws up once in a while. It supposedly always gets fixed, but that bug has been lingering around for about a year if his thread comments are any indication.