Gee, you'd almost think that someone at Runic was reading this thread....
Anywho, my thoughts:
1. I could not tell from the video if the berserker's charge was building up based on "hits landed" or "damage dealt." The difference is hugely important because a "charge up per hits landed" mechanic is going to prove virtually unbalanceable. The traditional tradeoff in action games of all stripes (including ARPGs) is that attack speed and damage per hit are inversely correlated. (ie Things like hammers deal huge damage but swing slowly; Things like daggers swing quickly, but deal low damage. Overall DPS works out the same either way (or at least it's supposed to if the game is balanced properly).) However, a charge bar that filled on a per-hit basis and awarded extra critical hits would directly correlate damage per hit to attack speed. This would push berserker build optimality into a 1-peaked configuration centered around the fastest possible attack speed, leaving everything else as "neat, but too slow to be any good."
This, btw, is quite similar to what happened with TL1 enchanting allowing you to arbitrarily increase damage per hit (but not do much about attack speed) of any weapon -- base attack speed became the be-all-end-all of weapon stats. (Aside to Runic, please
make sure that enchanting does not return in anything even remotely resembling its TL1 form.)
The other classes are going to have pretty much the same issue with a "hits landed" charge-up system.
So, to sum up, charging should be based on "damage dealt." [but see below]
(Self-contradictory thought: A "damage dealt" charge up system passes through the unpredictability of the damage system, which can make it hard to play the system tactically. A "hits landed" charge-up system is totally predictable and therefore lends itself to tactical use. So, pick your poison: a charge-up system that unbalances everything in favor of max attack speed or one that is hard to make intelligent use of.)
[Actually..... the more I think about that, the more I feel that (1) being tactical-use-friendly may well be the more important issue, so perhaps a "hit based" charge-up system would be better; and (2) the fact that you have to break something either way takes me back to wondering if maybe the best answer of all is no charge-up system.]
2. The ease of charging/overall uptime ratio seen in the video seemed... well... just about right. It looked like you could simply ignore the whole charging business and treat it like a baseline DPS boost inherent to the class; OR approach the charging up tactically (timing and target selection conscious of the charge bar) and reap a slightly higher practical DPS when and where it matters; with the difference between the two roughly in proportion to the added costs in terms of concentration and repositioning/holding back. Either Runic tested this out or they got really lucky.
At least for the berserker.
If the outlander is merely getting an IAS/IMS boost, it should work about the same as the berserker.
I'm most worried about the embermage. Free spells with fast casting sounds a lot more potent than auto crits. Where the berserker seems (as best one can judge from a single video) to have hit the sweet spot between the charge-up mechanic being pointless and being all-important, the embermage may tip over into having a charge-up mechanic that's all-important.
Sounds like the engineer is going to play like a D2 Phoenix Strike Sin -- charge up, release, lather, rinse, repeat. That's fine I guess, but I would like to see non-charge-up-based builds being equally viable with charge-up-based builds. Or at least a variety of finisher moves to avoid a situation where there's really only one viable engineer build.