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 would probably say out of universe they could not make a full episode on planets that had nothing on them. They could only do so much with an empty planet. Whereas class M/Earth like planets they could introduce new species easily. In universe answer I would say that they did but because the vessels in the episodes were not science vessels they were never tasked with such a mission. The Enterprise was the flag ship for the federation, I would probably say combat and diplomatic missions were more important for that vessel. That is my theory.
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.