If critical hits remain as they are, not being able to critical is a nice minor penalty (for prone perhaps)
I like this...
and perhaps we could offer criticals on 19 or 20 (for bless perhaps).
But not this.
Honestly, I feel that always critting on a 20 is probably a bit too frequent. In principle, I would prefer a confirmation roll be required. However, I'll gladly accept that I'm in the minority on that one. Besides, rolling a nat-20 is cool.
However, because this means that crits will actually be reasonably common, I'm inclined to think that they should only give a moderate bonus (4e's auto-max damage was good; the weapons that added extra dice on top of this were too much, IMO).
And I would certainly oppose any measure to have crits occur on anything other than a nat-20 - in retrospect, I think the expanded threat ranges in 3e were a mistake, even with the confirmation roll.
Rather than apply disadvantage, you could not be able to benefit from advantage (and vice-versa - not being subject to disadvantage seems like a good benefit for a mob).
Again, I think this is an area where WotC are making a mistake. IMO, advantage and disadvantage should simply cancel one another out, rather than the current situation where you only roll 1 die, but otherwise have both. (So, no Sneak Attack for the invisible, blind Rogue, please.)
For group effects, to saves perhaps, you could use the best or worst save bonus from the group rather than just a numerical effect.
I like it. I would be inclined to go with the best bonus, but apply a "drag factor" for the rest.
That way, the sneaky Rogue can guide the party stealthily through the enemy stronghold, rather than have his efforts outright destroyed by the noisy Fighter. Otherwise, if it goes by the worst (or everyone rolls), you quickly see stealth becoming a non-option due to the noisy Fighter, diplomacy being negated by the tactles Barbarian... and the party is left with only the common denominator of fighting their way through every adventure.
For the healer speciality, you could step up HD rather than maximise them.
I like this mechanic, and not just for healers - IIRC, the first playtest packet had some sort of implicit weapon specialisation mechanic that increased the damage die by a step. Also, bonuses to the damage die (or similar) are much easier to balance for than bonuses on the attack roll - the one is a point or two extra damage, while the latter is potentially tens of points from extra hits.
Overall, I don't mind the odd bonus, but they should *never* stack - the highest magical bonus to any given dice roll or defence should apply.
I'm broadly in agreement. I might be tempted to have both a semi-permanent 'condition' modifier and also an instantaneous 'power' modifier in play (where the former applies to all d20 rolls, while the latter only ever applies to one specific roll, taken immediately).