When you're first starting out, positioning matters. At http://yesgraph.com we've found copy AB tests to produce incredible lift.
For example, people don't want to "invite" contacts. They do want to "email" contacts though. It's the same flow, but a few words triggered massive lift. The reason it is massive is because it is so unoptimized. So it is specifically at the start where such small tests can matter.
However, you only have so many "bullets" to shoot at tests with a small audience so you have to be very picky. Sounds like contact inviting is a key viral feature of YesGraph so it makes a lot of sense to optimize there.
I'm not anti A/B testing at all, but anecdotally a lot of people I talk to about this stuff fire their test bullets on the wrong things and end up with not much to show for it.
A better understanding of psychology goes a long way into intuiting where you may actually be able to see gains, and how to go about achieving them. Sometimes this can be small changes producing large gains, although I have seen large changes produce larger gains more often.
Dan Siroke, CEO of Optimizely always, always stresses in his presentations it is vital to test. The presentations on his SlideShare are awesome: http://www.slideshare.net/dsiroker