Generally 70% is the lowest level of dark chocolate you'll find in any great quantities. Of course there are many 65% bars, but seventy per cent is where the serious action starts to take place and chocolate makers begin to explore an enhance the core cocoa flavour.
It should be noted, that it a common miss-perception that all 70% dark chocolate bars are essentially the same, for they can be radically different. If you take the Askinosie del Tambo and the Daintree Estates Dark. The former is very brutish whilst the later relatively sweet and tangy. There are also a wide variety intensities offered by Domori - all of which have fairly noticeable creamy textures.
You will often find chocolate makers combine 70% chocolate with a variety of flavours. Often they'll add salt which brings out the natural flavour of the cacao or other ingredients such as fruits, salt and cocoa nibs.