’Accessible’ - What does it mean in Prog?

If you define prog as "aquired taste", then you might use a term such as accessible to describe music that is easy on the ear - that being instant recognition of melody, harmony and rythem in a known structure (verse and chorus style) Any likness to mainstream/pop music which most of us know very well, will make it more accessible. If you have heard little music outside of the big radiostations, MTV etc. then prog could be described as inaccessible.

Accessible prog: Pink Floyd, Marillion, Genesis as opposed to King Crimson, ELP and Bruford to mention a few.

Further to recent exchanges of view, I seek opinions out there about the use of the word 'accessible' to describe strands of prog.  I've always been very partisan about the kinds of music I like, what instruments are involved, what moods are created, how interesting a musical passage or phrase is, how complex it is etc.  On numerous reviews for this site, I notice people using the words 'accessible', 'inaccessbile', or 'least accessible', and I wondered what they meant by that.  I never had to go through Emerson Lake & Palmer to get to Henry Cow (one of the bands tarred with this brush), or work my way through The Moody Blues to get to Magma (yet another). 

Surely reactions to music are visceral and instinctive, and its more a question of appreciating specific elements within the sound, rather than responding to progressive (sic) levels of difficulty or mood? 

Answers on a postcard please, preferably with examples to show me why we need to warn potential new prog fans away from especially dangerous and inaccessible areas.