The response rate. People like to laugh at social scientists, but they tend to get very iffy about a survey when less than 70% respond, and they'd regard something with a response rate of lower than 50% as pure garbage unless there were very good reasons to believe that the non-responses were not affecting the sample. And yet, we had polls with response rates between 16% and 65% (and averaging around a third) published and broadcast as if they were gospel.
Not that this is entirely the media's fault - polling companies tended not to report the response rate (the TNS polls of the Maori electorates being a notable exception). And the reason for this is pretty obvious - because if would make it perfectly clear that they were peddling snake-oil. And OTOH, given the Herald's headline - "Erratic election polls blamed on sample size" - you really have to wonder whether they'd notice.