USS Triumphant wrote:
I don't *think* anything in canon has established that what makes a civilization worthy is necessarily warp drive - warp drive (or presumably any other fast interstellar transport system) is just a dead giveaway that the PD is moot for that civilization, because they are now out looking for other civilizations to interfere with them.
A few of the novels have said that there is a complex system of civilization analysis involved that gives a civ a score and determines if they are ready for contact or not, and interstellar drive is only one factor in calculating that score among others such as those you mentioned here. I like that concept.
The link above is pretty exhaustive in an attempt to track down the examples of observing and not interfering with pre-warp civilizations. The major exception has to do with the Omega Molecule and suspending the Prime Directive due to urgency.