To me, Opeth's song length is a major factor in my perception of how great they are.
How many times have you watched a movie, and hoped for it to go on? Opeth gives me songs that by the end of my listening, I am satisfied. My usual complaint with musicians these days is how they slap out an album that's 35-45 minutes in length, with songs that are unimaginative. When I listen to Opeth, I feel that the band is not in a hurry. I don't feel a sense of being rushed in their music. They take their time moving through progressions, and even though some may complain about "over done" song sections, I find it satisfying. The 2+ minute outro to Deliverance for example: it could of been done with 2 refrains, but they chose more repetition, which just gives me more time to enjoy it.
Truthfully, until Opeth came along, I seldom enjoyed the longer "epic" songs I've heard. So many long songs just seem to be a waste of air space. But Opeth pulls it off to near perfection. I don;'t think I've ever commented to myself that a particular song could of been shorter.
It's fun to see how each person perceives a band. There are so many varied opinions in what Opeth does to people, and how they break-down the components of Opeth. I generally don't spend time over-analyzing the music I listen to. I either like it or not, and not for any real musical reason - my ears do my work for me.
I like as many simply structured songs as I like complicated ones. I just find that overall, Opeth creates the most mesmerizing music I've ever heard for the first 43 years of my life, and once I start listening to them, I donm't want it to end. So I personally hail their ability to continually create epic length songs that 110% keep my interest.