For me it's easily the most interesting feature coming (hopping bird animations not withstanding), so I thought I'd get a hype thread going.
Here's the vid with the most detail, for anyone who missed it. Obviously a lot we still don't know, and everything's still subject to change (and going by recent form, as much after release as before it). But probably fair to assume that the designers had the classes roughly where they wanted them by v3, so other than the tweaks necessary to suit them to a the multi-class system I doubt there will be much change, at least in terms of fundamental mechanics or class-defining abilities. Which is a roundabout way of saying I'm going to speculate madly and nobody can stop me.
The 75-85% rule seems intended to keep wonky builds viable, so in that spirit I'm thinking of which classes could work as a secondary with just a handful of levels, ending up 17/3 or even 19/1, letting you change up the playstyle and still get most of the high-level goodies available to the primary class.
Just going through the list:
Barbarian - seems like the ultimate one-point wonder for any melee class, just for carnage. I suspect there will be changes there, eg spreading on-hit effects requires a separate talent further up the barb tree.
Chanter - great for action economy, as it all goes on in the background. Maybe limited to weapon users, if it turns out spellcasting interrupts chants (makes sense that it would). Usefulness will depend on where the key phrases are - if eg ila's sure arrows is still at 3, it could be well worth for a ranged dps to grab it just as a fire-and-forget buff, never mind the skellies.
Cipher - similarly good economy for any weapon-user, as focus will generate naturally. I imagine the problem would be more in discharging it fast enough to stop it capping out and turning off the whip, since you'll only have the cheap powers available
Druid - depends how well shapeshift scales - could be a neat choice for ranged chars who need a way to delete the odd diver in the backline. If it's just a touch too weak though it becomes kind of a deathtrap. Won't have much time for casting, but those beast control spells could be handy
Fighter - has been in kind of a weird spot since they upgraded the AI to exploit their one weakness: not really being able to do much but stand there and bop things. Probably the best candidate for changes, i guess we'll see. If extra engagements become more useful, could dip in to grab defender for a frontliner, i suppose
Monk - good economy, as wounds generate themselves and abilities that aren't passive tend to be fast. If there's any good CC or self-buffs at early levels it could be a worthy pickup for a frontliner, or anyone who plans to get hit a lot. I dunno though, I feel like monk synergizes with itself so well, it would be kind of a waste to only take a splash of it
Paladin - doesn't seem like it would work that well. Aura is nice I suppose, but the defense boosts take more levels to accumulate, and most of the good stuff arrives later. I imagine it will be one of the better classes to make proper hybrids out of though, since they have buckets of utility and their abilities just work, there's no faffing around with resources or conditions.
Priest - could be worth it for the god-specific accuracy bonus, and even a downscaled radiance would see plenty of use. Probably won't have much time to cast spells, though, except maybe frantic minor heals in desperate moments. Also god-related stuff in dialogues and the disposition juggling minigame, those are fun. That goes for pallys too. How priest/paladins work will be interesting to see
Ranger - depends how the companion scales, but probably not that useful without investment
Rogue - sneak attack, possibly escape - worth considering for any damage dealer I would've thought
Wizard - wand-related abilities I guess, if you're making some funky blaster that isn't a wizard to begin with. Otherwise, same problem with opportunity cost as other casters - it's a lot of time to spend doing something you're not very good at.
So, there's that. Probably not worth putting that much thought into it when there's still so much to be revealed. For all we know they might have chopped up those class-defining abilities and spread them around to prevent exactly the sort of thing I'm talking about. Oh well, too late now.