I can already tell you Reaver can generate guard. They have Block and Slash, it's upgrade generates 15% guard. Every skill in vanguard except 1 of them generates guard. Charging Bull, both taunts and Unbowed. Ultimately it depends what abilities the Reaver gets 'besides' the 3 Reaver skills. If the last stream was any indication each class has 12 active abilities to pick from (6 in each tab) and 20 passives (10 per tab). So... probably a good bit of wiggle room, and I'm sure there is at least 1 other guard builder then just block n attack. But then a move that nullifies a hit completely and builds some guard isn't such a bad thing.
As far as the 2 abilities we 100% know about (well, for base). Area of Pain seems to add 200% weapon dmg, consumes 10 stamina per second and is a toggled ability. Not sure if that's 200% dmg per second to everything in the area, or if it's just adding +200% weapon dmg to EVERY attack you do. If it's to every attack, auto-attacks become pretty deadly (I mean 300% weapon dmg? yus prees).
Devour is only 100% dmg, but does 20% bonus dmg per 1% of life missing, so at half health your looking at a pretty solid 1100% weapon dmg. It has a secondary bonus effect of healing you for (and this is hilarious how its written) 1% your life per 1% missing life. Or you know, in less words - full heal - as long as you use devour on an enemy currently effected by your Ring of Pain.
Third Reaver skill I don't have any hard information on except 1 tiny little detail.. it's a 3 hit combo (or just has 3 hit chain if used in succession) and doesn't consume stamina. It consumes health. So if you where able to build solid guard, you could use this to get your health lower to bulk up devour, and combo pain + devour for a full heal before guard breaks.
That all said, Devour cost like 65 stamina which is absurdly high cost compared to all the other skills (most are 35). Block and Attack is 10 to 'start' but you can keep holding it and it'll only drain 5 per second. Does 150% weapon dmg, I think +50% and the 15% guard when upgrade. If you did that inside a Ring of Pain you'd be consuming 15 per second past the initial cost and do like 400% weapon dmg each time you blocked to gain guard back at your attacker... not sure how sustainable that would be and or how useful against multiple enemies.
Outside of the 2H counter move.. Reavers might only have other 2H skills and Battlemaster stuff which would heavily limit there ability to generate guard... but I haven't heard any actual evidence towards any of that. All I know are the 4 skills on the class card thing. Which is you know, 4 of 12 so that's a good 8 skills on there list just kinda missing, well, 7 since we can assume the 3rd reaver skill will be in there. Course, Mighty Blow and Counter is 2 out of 5... so that leaves 3 from 2H that could be in there and... yeah, they could totally have no vanguard abilities. Somehow I doubt Reaver completely lacks guard builders though outside of the Block n Attack move.
-edit-
Actually from the MP video I'm pretty confident Reaver also has the earth shattery BS move from 2H tree too. Good chance she just has all the 2H skills. Not that it matters 'to' much. Your ultimately stuck with only 4 skills you can possibly bind. I'd imagine each warrior has a taunt though, would seem odd to have a warrior with out a taunt, and all taunts build guard at base. Oh and we've seen weapons that build guard per hit, forgot about that. But then that was a lvl 4 char building 10 per hit... not exactly a ton but hey it's something!