I don't know why everyone says they sound like Opeth. Seriously, in many cases you have to really listen deep and make stretches to hear any similarities, and at least with the last few albums I don't hear Opeth except vaguely once or twice in the entire disc. To me that doesn't make the bands similar.
For example, I don't consider Opeth like Megadeth yet there are a few select places (the solo, solo riff, and especially riff in between the solos in White Cluster) where Opeth sound like Megadeth.
Katatonia's first few are doom, and their last few albums are a little more atmospheric, it is very accessible, the vocals are clean everywhere (except one or two backing tracks). The production on Viva Emptiness far outstrips that of Last Fair Deal Gone Down, and of newer Katatonia I would definitely recommend Viva Emptiness. Katatonia has also become a perfect example of a band which could so easily break out of the 'underground' if they had support. Honestly if Katatonia were on RR, "Criminals" which is a great song could easily be a prominent radio hit. They've pretty much become alt-metal with some progressive elements, with an incredible weight and gloom to their music. Very much an atmosphere of depression. I really dig Katatonia.
Oh, one more glaring diff. between Opeth and Katatonia is Katatonia increasingly has the verse-chorus-verse structure in a lot of their songs, very straightforward in terms of sequencing.
Incidentally, if you go to
www.katatonia.com and go to the 'Releases' section you can download samples of tracks from most of their albums. I would suggest listening to all the samples, they do a decent job of letting you hear good portions of the song (the Criminals sample for example features the first vers, pre, and then most of the chorus). I suggest you also check out Anathema's "A Natural Disaster".
I'm not even going to touch that 'thrash' comment.