Fast-paced environment usually describes places that lack an attention to detail and quality, which above anything else should be the core focus of a company. A lot of people are suggesting that pace refers to the amount of work which I don't think is entirely correct. A business could have all the work in the world and still move at a snail's pace. @mrspeaker made an excellent point in that the ideal environment would be "well-paced." More often than not, though, this sort of terminology was probably picked from similar job postings that the HR person (generalizing, of course) saw elsewhere.