For someone with a well-trained metal ear, of course the albums don't all sound the same. However, I don't think anyone can make a case for a vast progression between each album. Mikael himself said that there would never be a huge change in style. The only real significant change in songwriting and sound was My Arms, Your Hearse as opposed to Morningrise. Since MAYH there have been few big changes. Compare this to a band like Ulver, who have been all over the place, from harsh black metal to folk to electronic minimalism. I think Opeth have changed their music just enough to keep most people satisfied that they were growing as musicians and songwriters. Needless to say, their musicianship has improved greatly in this time, which might account for some of the "change in sound". The bottom line, though, is that Opeth is a conservative band. Having said this, I must profess that I am drooling in anticipation of Damnation, perhaps their biggest step out of the box.
And yes, I've been an Opeth fan since '97, so I'm not one of the 'quick to judge' types.