Adverts with with a tight specific message get clicked more often.
Give the searcher what she wants. More signal, less noise. More needle less haystack.
Use your 130 characters* to reinforce the only real promise your ad can make: "I understand what you want. If you click here you will find what you're after."
There is not enough space to waste it talking about all the other wonderful things you do. That's for later.
* A Google text ad is limited to 130 characters. 25 in the headline and 35 in each of the other 3 lines. [Update: Google changed the format of the ads in 2016 to allow for longer adverts. The principle remains the same.]