As I recently told Dalek, the planet with the "primative architecture" that we've seen, could be considered to be belonging to a society in a Renaissance-era development. In my example, I used the Ba'ku society. They have warp capability, but choose to live more primatively, to avoid creating similar mistakes that occured on their previous planet.
For STO, I would hope our away missions lead us to a wide variety of planets, and not just a bunch of re-hashes of Earth. This is one thing I never liked about TOS, where many of the planets looked almost exactly like Earth, both in appearance as well as it's inhabitants.
"In this galaxy there's a mathematical probability of three million Earth type planets, and in all the universe three million million galaxies like this, and in all of that, and perhaps more, only one of each of us." - Leonard "Bones" McCoy
I fully expect there will be a vast amount of Class M planets for us to explore, but there should also be planets of other types, such as the ice planets, water planets, desert planets, gas giants (thou not explored by ground =P)