Every writing course I attended in grad school told us that the first sentence of the abstract should be understandable by any scientist, the second sentence should be comprehensible to a scientist in a related field. After that, you address the specialists community. However, it seems that more and more people are skipping the first and second sentences and jumping directly to talking to specialists. I have mixed feelings about that. On one hand, it's nice to skip the generalities but at the same time some times its nice to have some context.