TLDR: Breaking a BB streak reduces your streak count incrementally, rather than completely wiping it
I like the recent changes to XP - its making casual play much less penalized compared to grinding Shadow Crypt/Litany/Vale/VON ad nauseum. I like the proposal for the XP curve, where you're not stuck at 18 for half your life. There's just one more component, I think, that needs to be addressed to help penalize casual play a little less: BB streaks.
I think the problem is the overwhelming penalty for breaking a BB streak - you lose ALL your progress if you complete just ONE quest off-streak. You have to go do 5 more unique first-time quests to earn back your streak; if you're at endgame (L18, for a TR life), where running quests on Elite becomes much more challenging, and where the number of quests is already limited, that's pretty much a dealbreaker, and you just wont run a quest at all if you cant do it on Elite, at least not till you've out-leveled it for BB. I think anytime a system encourages you NOT to play, its detrimental to the game.
I'd like to see the penalty drawn back a bit; completing a quest first-time on Hard will cause you to lose 1 count of your Elite BB streak (so, from 5 to 4). Completing a quest on Normal will cause you to lose 1 count of your Hard BB streak, and 2 counts of your Elite BB streak. Completing on Casual will still completely wipe your BB streak, because, come on...
That way, you still pay a penalty if you have to drop down a difficulty for one quest, but its not so onerous that you wont every pay it to 'unlock' a quest for re-running with your daily playthrough bonus. It'll also reduce the disincentive for people to open/join PUGs on anything other than Elite...people will still run Elite for BB, of course, but if all that's there for a quest you need is a Hard run, you wont necessarily discount it out-of-hand, so it will increase socialization a bit.
(Conversely, if you don't like that idea, instead of reducing your "real" count by one, it could reduce your "extra" count by 5/10 and 10/20, respectively - ie, if your streak is really 60 quests on Elite, and you run one on Normal, you lose 10 or 20 counts, and you're down to 50 or 40. Basically, then every 10 or 20 quests you run on Elite after your BB is maxed earn you one "mulligan" you can use to "skip" a quest with no penalty to your actual BB bonus)