The local maximum is a point in which you’ve hit the limit of the current design…it is as effective as its ever going to be in its current incarnation. Even if you make 100 tweaks you can only get so much improvement; it is as effective as its ever going to be on its current structural foundation.
Joshua Porter discusses the idea of the local maximum, which he traces to Andrew Chen's article on becoming a metrics-driven business. Both write of how to move beyond the local maxima. Both see this as a result of concentrating on quantitative versus qualitative data (e.g. interviews, usability testing, ethnographic research, etc.).
There are two points I'd like to make about this. The first is that the qualitative research is about what I often refer to as "zooming out." Of course, better minds than mine have referred to it as considering the next larger context. Talking to other people, the people that actually use or might use your product radically changes your perspective.
The other point is that this can be painful. Seth Godin wrote a number of years ago about the pain of getting to "Big Max":
The problem is that to get to Big Max, you need to go through step C, which is a horrible and scary place to be.
Having your perspective radically altered can mean that you need to make radical alterations. That's never a very comfortable place to be.