The product of improvisation is the process of improvising.
Don't compare improvised theatre with scripted theatre on depth or layers of the story. Also, don't compare improv comedy with standup comedy on the intricateness of the jokes. When compared according to their rules, the scripted artforms will usually be better. As in music, nobody improvises a Beethoven symphony.
When evaluating improv, look at the product of improv, which is the very process of improvising. Let the audience enjoy and appreciate the fact that you're improvising: go towards the danger, embrace the failures and enjoy your time on stage to the fullest.
Have you ever noticed that after a scripted show, or during the break, people always talk about whatever is on their mind in their lives, but that during or after improv shows, people always discuss the show and what they just experienced?
That illustrates the difference between a result-based performance and a process-based performance. As an audience member, your trade-off for getting less polished results on stage is that you are getting real-time insight in the discovery process!
This is the very reason that I'm really not so happy when people approach me after a show with the words: "Waw! You really couldn't tell it was improvised!". Sure, it's meant as a compliment, but is it really one?