In the recent thread on SI's new orchestral library, someone mentioned that a) the prices need to come down and b) it's all about the programming.
The programming comment was a particularly important one. I recently downloaded RealGuitar and started playing around with it. The samples are relatively clean, sound good, but what REALLY sets this sample set apart is the programming.
It's much more than simple keyswitching. The way the programmers have laid out the keyboard and its functions allows you to create unbelievably realistic guitar simulations.
That kind of innovation is what the sample world needs. As great as I think GPO and other well programmed libraries are, I'd love to see developers take it even farther. In fact, if I were a developer myself, I'd be calling the guy who programmed RealGuitar as we speak to see if I could get his involvement on the next project.