I should start by saying that I have zero experience on a disc priest in Legion or any other expansion, so the thoughts I have about disc aren't necessarily specific to BFA. Some of this stuff may already exist on live and I've just been totally ignorant of it. But what has drawn me to disc in the BFA alpha is the spec's use of absorbs and damage-as-healing.
Disc on alpha has shifted back to the “spam
Parola del Potere: Scudo” role that the spec has become known for. While I'm sure this will be strong for raiding (because absorbs are always strong for raiding), this will be a very interesting healing style in M+ because no healing spec in Legion had access to an absorb with no cooldown. One of the primary limiting factors in pushing keys as high as possible on live is the ability to survive boss mechanics that hit hard. Various boss abilities would start dealing so much damage that you needed more mitigation or health than you could provide by yourself, and external defensives like
Pelle di Legnoduro or
Benedizione del Sacrificio have a long enough cooldown that they couldn't be relied upon to cover every cast. If a healer in Legion could put an absorb on all party members before abilities like
Fulmine Concentrato or
Contusione Malvagia, that healer spec would have an enormous advantage over its peers in pushing keys. So the change to
Parola del Potere: Scudo is potentially meta-warping for high keys.
The other aspect of disc that I'm really enjoying is its ability to heal by dealing damage. One of the reasons I've really enjoyed resto druid in Legion is that druids can deal significant damage if they've got the correct legendaries and talents. Unfortunately the removal of legendaries in BFA will likely mean that druid damage falls off significantly compared to Legion. While I still enjoy the druid HoT-oriented healing style, I'd like a healing class that can focus on dealing damage when needed -- I'm not sure druids will be able to do that effectively in BFA yet. One of the things about disc that is very appealing is that its mastery increases the damage dealt by
Contrizione. I'm not sure how strong mastery will be for M+, but being able to have all your secondary stats contribute to DPS is something that I believe is sorely needed for all healers. Currently no healer benefits from mastery in terms of DPS, and many of my equipment sets on live have as little mastery as possible, i.e., only the mastery I get from my weapon and relics.
Disc still seems a little squishy so I'm not sure the class will be viable in high keys, but it's definitely fun to play and offers a unique kit that could be very powerful.