Well ,
Orchid was more melodic that the other albums , and less emotional than Morningrise
Morningrise was like Orchid in the sense of production and arrangement , except it was a step up in emotionality (is that a word)
Mayh was the biggest change in Opeth history , there wasn't a riff on there that could of been on the previous albums and vise versa . The vocals are better as with the production .
Still Life is not as big of a change , the album's emotionality level is different from the previous ones , vocals and production are much much better , can't really explain this album much (concider the fact that I don't have it )
Blackwater Park is a step down from Still Life in my opinion , yet in some areas it's a big step up , the only thing that has improved on this albums is it's "groovyness" fell , vocals , vocal lines , and guitar effects , also the drumming . (hmm I geuss it is better)
Here is my opinion on what the next album will sound like . For the next masterpiece Opeth will try something new , eveything will be different , Opeth will succed in this and attract new fans yet also overly impressing there old diehard (and not so diehard) fans , it will be a big step up , and it will be opeth's most expieremental album .