Question: Starfleet is supposed to "explore *strange* new worlds and boldly go where no man has gone before", yet most of the time they explore worlds that are more Earth-like than not, and are inhabited by human-like species.

I understand that the out-of-universe reason is that it is difficult to tell a story when the characters are always in space suits, but couldn't the writers have found some other way, rather than skipping all the interesting non-humanoid life forms? I remember the animated series characters using a skin-tight force field (a life-belt?) to explore inhospitable environments.

Or, are there really not that many interesting life forms on other classes of planets?

Answer: 
I think we are meant to make the assumption that intelligent life in the Alpha quadrant (at least in a form recognizable to humans) is typically humanoid and therefore typically requires an M-class environment. This idea is supported by the TNG episode that touches on the existence of an ancient race of which most known species are descendants ([The Chase](http://en.wikipedia.org/wiki/The_Chase_%28Star_Trek:_The_Next_Generation%29)). But there are exceptional species, e.g. the [Benzites](http://en.memory-alpha.org/wiki/Benzite).
Personally, I don't think they want to explore the worlds. I think they want to meet the people. Humans are social creatures. We saw several times in ST:ENT where Archer chose to go with an inhabited world instead of a astronomical oddity. When they explored that star that was close to supernova, they seemed more interested in meeting the people who were studying it.