I've noticed the Opeth-bashing (both on metalarchives.com and whereever else it occurs) usually hits on 3 main subjects:
1. Opeth's song structures (The critics have a problem with the way Opeth sporadically and constantly switch between acoustics and heavy guitars in many of their songs. I don't blame them... this was one reason it took me awhile fully get into the album, Morniningrise, and much of Opeth's music, in general.)
2. Opeth's influences and how they incorporate it into the music (According to what many of the critics say, Opeth's music is not innovative in any way, since it derives alot from 70's prog, early 90's death/doom, melodic metal, etc. Typically, I've seen people (when in relation to this subject) say Opeth rips off bands like Amorphis, Camel, Katatonia, etc.)
3. What alot of Opeth's diehard fans say about the band (This pisses off the critics the most. Opeth has definitely become one of the most popular and well-liked metal bands (within the bounds of the metal community, of course). This led to a great deal of fanboyism, which has spawned comments such as "Opeth is the most innovative band ever!" or "Nobody can compare with Opeth... Nobody!" The critics then take that into account and either denounce much is what is said from these certain diehard fans or just make a full mockery out of both them and the band)
Summing up this post, I would definitely say Opeth is definitely one of my favorite bands, and when I hear critique about them... I usually don't mind at all as long as it is done in a legitimate, intelligent, and coherent way. It only gets annoying when someone starts saying shit like "Opeth sucks! They have about as much worth as cow dung" or conjures up these ridiculously asinine insults aimed at the band (ex. "Blopeth"... wow, how fucking original and creative is that!?!). Then they may go on and say how Opeth can't write riffs or something. It's fine if you don't like the band, but to ramble on about how they suck or how they have no talent is ridiculous.