Stealth.
Hiding.
If I duck behind some blocks-line-of-sight object then pop back out, do I need to take the Hide Action - and pretty much end my turn? Or do I still have an action available?
Can I get the benefits of 'a sneak attack' or 'a surprise attack' (with no capital letters) when I come back out from behind it?
5e Surprised condition vs. 4e Surprise Round. (Full disclosure: I use the latter in my 5e game. If I ever have an Assassin in my group, I'll have to learn the former.)
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)
I have seen two mentions of player knowledge vs character knowledge. While I agree this is both assumed and enforced as needed, I don't know where this is specifically stated in the rules. This post is about RAW. The fact is lots and lots of player knowledge is assumed to also be character knowledge. Players know a lot about the pantheons, physics, magic spell effects, movement speed, or even distances which might not be immediately apparent to a character. For example, in real life, if there was a person standing 25 feet away from you, you wouldn't necessarily know if they were 20, 25, or 30 feet away. However using minis you can count pretty quick, and you know most humanoids only move 30ft, so you might attack, then move 10 or 15 feet back so that they have to double move to get in your melee range. Without rolling for knowledge and keeping a detailed list on every possible subject, you don't know what your characters are fully aware of.
Basically I just think the topic of meta-gaming, or using personal knowledge for the benefit of characters is beyond the scope of a RAW discussion. But I could be wrong and it might be very plainly and flatly written that players aren't allowed to use their knowledge of the game.