To be fair, I've only been listening to Opeth for about a year, so those of you who discount my opinion based on my length of association with the band can stop reading now.
I find their early efforts to be somewhat uninspired and sophomoric, though enjoyable. Orchid, Morningrise, and My Arms, Your Hearse sound like death metal-infused power metal. The band hasn't figured out their exact sound yet, but they've clearly got something interesting going on. They figured it out on Still Life, perfected their sound on Blackwater Park, deviated slightly with Deliverance and Damnation (both of which would have made one great album if cut in half and combined), and finally realized their songwriting prowess on Ghost Reveries (an absolute treasure). Watershed is a natural evolution of Ghost Reveries, and a very good album. Heritage is, like Deliverance and Damnation before it, an experiment. People are mostly upset because this experiment may become the future evolution of the band, and that challenges their beloved first impression of Opeth. Oh well. Their music is good no matter what.