My personal experience with this subject leaves me with a simple answer to the posed question. A GOOD 3D design is measured by it's performance BETWEEN the unstalled, stable flying state and the completely stalled, stationary flying state(hover). The worst designs show their flaws here in between those two states. Desings that are too stabele tend to snap and jerk around while those that are unstable will not want to fly well in the tradional manner. Sure you can hover a trainer but how much fun is it getting it into and out of that hover? The benchmark maneuver for all of this is the harrier, if the plane won'rt harrier it will be hard to enter and exit ANY high alpha state and as such becomes a poor 3D design....think UCD and you'll understand me. Also, when I say harrier I mean nose high, slowed down to a crawl and completely under control all the time. If your plane has a sweet spot you need to get it into to perform the maneuver it's not a good 3D design IMO.

In a nutshell its the transitional characteristics that make for a good 3D design.