In the case of Anathema, only their very early stuff was like Death Metal. Once they changed vocalist, they quickly evolved from metal to rock, before they became really popular and well known.
Opeth have just changed too drastically, too quickly, after they had already established themselves as a popular and well-known band. They've lost the heaviness so much that they really should have called it quits and started with a new name. At least with Metallica, they were still heavy, even if the sound became more cleaner and polished.
I actually just listened to "Pale Communion" for the first time after making my post yesterday, having assumed it was going to be another snooze-fest like "Heritage". It actually wasn't as bad as I was expecting, and could probably fit amongst my prog rock collection. But imagine if it was Metalica playing "Pale Communion"... you'd still have a metal sound with big guitars, big drums, and a metal-sounding vocalist, instead of the wet limp-wristed sound presented by Opeth.