1) If everyone rolls their dice out in the open (as we do), its pretty easy for players to work out ACs, attack bonuses, hit points, ability modifiers, etc.
It can be a good idea for the DM to keep his rolls behind the screen, of course. But everyone else, I should hope, out in the open, sure.
And I suppose it's a matter of taste whether working out the game stats of monsters/NPCs is "metagaming" or "immersion."
As a DM I am happy to have the players be familiar with the stats of commonly encountered schlubs. When presented with a patrol of 20 soldiers, they will have a good idea of whether they should take them on or not.
This allows me to present a more naturally diverse set of encounters with variable difficulty. The players can decide for themselves whether they should run, be sneaky, negotiate, or just charge in. Of course occasionally, things ain't what they seem . . .
Some systems lend themselves better to stats out in the open than others, IMHO. D&D, traditionally - and 5e is traditional D&D - works well with the DM keeping plenty of info behind the screen.
But that, too, gets into the realm of personal preference.
And the DM didn't want to be nasty, he wanted to use lower CR creatures in number to try this "bounded accuracy lets mooks be effective, yada yada yada."
BA delivers mooks still being a challenge and overleveled horrors still being touchable, to an extent. But it has some stumbling blocks. Numbers can tell too much under the wrong circumstances - the mooks can suddenly become an overwhelming threat, not just the credible one intended. Save:1/2 can be a death sentence to an encounters worth of such mooks, undoing the point of designing in BA (they have a chance to save, even if not a good one, but 1/2 damage is still fatal). It replaced bonus inflation with hp inflation. Etc...
But compared to calculating hit% for hundreds of orcs in 1e, not s'bad.
Well, I don't expect the difference to be as much as it was in 3.5E and 4E, but more than they made it. Instead of a +1 improvement for prof bonus, a +3 or +4 would more represent the increased ability 7 levels of XP should warrant IMO.
Yeah, I don't think there is a 'right' answer for scaling bonuses to apply to a d20. 5e is tuned to use the small bonuses of BA. 4e was tuned to use rapidly scaling bonuses, but they were partially smoke & mirrors - as long as the DM used challenges very close to your level, that is.
Personally, I despise cantrips. But anyway, there wasn't the concentration nerf in 2E so I don't know why they did it now.
There were certainly AD&D spells that had durations based on concentration or couldn't readily be combined with other casting. So Concentration doesn't seem inappropriate, conceptually or in terms of tradition. It fits D&D magic fine.
And while the battle was well-in-hand I would still like to contribute more meaningfully then plinking away with cantrips or magic missile simply because of a game mechanic. Honestly, I could have had my character just sit down on a log and snack on popcorn for all the good she did while concentrating on slow.
You'd already made the most meaningful contribution of the encounter, so it hardly seems unfair for you to make attack rolls with your cantrips the same as everyone else is with their weapons...
...unless your cantrip forces a save, of course.