6. If you roll for wealth, you do not get the items from your background.
7. Backgrounds are completely customizable. By the default rules, players do not need a DM's permission to change a background's proficiencies.
I've only seen 6 as a conscious decision, and 7 is nearly impossible to convince some DMs of, no matter how explicit the PHB is.
Bonus action to drink a potion is one that I've seen come up a fair bit.
Active houserule in most cases I've seen it. Are you really seeing people claim it's RAW?
Rogues and monks can dash twice.
The game is balanced around 6-8 encounters per day.
And take a regular movement! or move, super jump, dash! It's pretty great.
The encounter one seems like a built in guidepost, not an actual rule, to me.
Acting on player knowledge not character knowledge. Example (from HotDQ):
My character sees the Rogue open a treasure chest, get a face-full of dust, run away screaming wildly, and jump in a lake.
My player knows the Rogue is hallucinating, is trying to swim down to the bottom of the lake, and is going to drown.
Can / should I jump in after and save the Rogue?
Casting a spell when you can't aim it properly (around corner / no line-of-sight, overhead at flying foe, through keyhole, terrain provides cover/concealment/both from each other)
Some of these seems strange, and about rulings rather than the rules as written.
WHy would the drowning rogue's ally need anything other than what they've seen to jump in after the rogue? At
most I could see requiring an Insight check, and even then only if the rogue is prone to wild gesticulation and random actions...but still, the other character saw the powder fly in his face, saw him yelling and panicking, and them jump into a lake.
If I saw that IRL, I'd jump in after my friend, after telling someone to call emergency services. If I weren't a good swimmer, I'd call and yell at the nearest fit person to jump in and get him. Obviously something is very wrong. Metagaming also isn't mentioned at all in the rules, so it's off topic.
Also, why would you not be able to cast a spell at a creature flying overhead? As for keyholes, if they're old fashioned enough...yeah, makes sense. At most I'd impose disadvantage.
To be fair, that's not at all clear without the Sage Advice ruling. Drinking a flask of water is absolutely covered by the Use an Object action, so it's perfectly natural to assume that drinking a potion of healing should be no different. There's certainly no in-game reason for that to be the case, and, frankly, given the power level of potions in the game it's really not a restriction that I would consider beneficial to retain. It's not at all clear that the individual rules for each magic item type or each individual magic item description are providing explicit exceptions to the normal combat action rules. If Mearls and Crawford had wanted the game to work this way, they should have added a Use a Magic Item action to the list in the PHB.
Until we got the DMG, I was doing this. After that, I houseruled it. I also allowed the tinkerer in the current campaign to make potion flasks that have easily popped off stoppers, with a flick of the thumb, so anyone can drink a potion quickly, and Rogues can do it as part of another action.
Mostly, I see stuff with the action economy, and people assuming spells work like they did in past editions.
And my group uses knowledge skills like they were in 4e, but I encourage that.