Question: What the heck are nested proficiencies?
Answer: I used the term nested proficiencies in a discussion and I should've known I'd get this question. It is shorthand for proficiencies which sneak into your character's background when you get other proficiencies, and which can either be added to the sheet or just assumed to be there by the DM. If you're using a program to generate your proficiency (or skill) list, these would just seem to magically appear.
An example would be the Engineering Proficiency, which automatically grants Maths.
There are four ways to handle nested proficiency conflicts:
The first way is to completely ignore them. I'm completely okay with this, because it's not something that's going to make a difference the vast majority of the time. If your character is solving a complex mathematical puzzle, I might ask what skills you're using to help, and Maths, Numerancy, Engineering, etc., would all be valid choices. This is part of the "More than one way to skin a cat." thing.
The second way, which I tend to use, is the idea that slots are SPENT when you get a proficiency, and can't come back. Therefore, if you have Maths, and take Engineering, you have Maths and Engineering on your sheet, and if you take Engineering, you can have Maths and Engineering on your sheet (and I would like it if you indicated you got maths free.)
The third way, which I DO NOT ALLOW in my own game, is slot rearranging. That is, your character spent a slot on Maths, then took Engineering, and therefore you now have an extra slot because Maths is now "Free."
The fourth way, which I only sometimes allow in my own game, is proficiency enhancing. In the example above, I might allow a +1 to the skill of Maths, or transmute your Maths into Higher Maths. I'm more likely to give this as a non-XP reward.
I'm going to try to predict two more questions, and answer them here:
Question: What is the More than One Way to Skin a Cat thing?
One of the recurring themes coming out of the group discussing DnDNext is the idea that perception needs to be its own stat. If we're going to replace using (for example) skills to discover a secret door, then just replace it all the way. One of the things I brought in was how I tend to resolve stuff in a room, which is to roll a handful of dice for the entire party, with the dice representing each character's best chance of spotting things, and then only being specific if they all or mostly fail.
For example, entering a room with a magical secret door, I rolled 2d6, 2d20 and percentiles. The party had two characters with a default elven chance of finding secret doors, two characters using skills to look for secret doors (one based on his incredible knowledge of masonry, one with his incredible knowledge of magic traps) and one follower of a god of luck who had a chance to just see anything automatically. If most of the scores succeed, I describe the door as barely hidden from the party. If only one succeeds, I tell that player the rest of the characters don't seem to see it. If they all fail, I roll any additional rolls before moving further ahead (for example, the elf is also a rogue, using find traps, so even if she failed the d6, she might make that roll.) [I roll this handful even when there are no secret doors, by the way, the party never knows why I am rolling.] The Ogre Magi fighter in the group has no chance of finding them at all, usually.
Either we're all using different ways to look for secret doors (for example) or we all have an innate ability to find such things. I can get behind either one. I can't get behind this innate ability being based on one, two, or even three stats. The more I play, the more I lean toward perception as its own stat.
Question: What are non-XP rewards?
At the conclusion of an adventure, the DM tends to give experience points. If the party is doing something for a king, powerful figure or a god, it is typical for that figure to give the party a boon. Non-XP rewards are doing this without the NPC.
I also use experience banking, which is when a player can put some of his experience into a bank, which can be spent (out of combat/game time) to work with other aspects of your character. Typical bank spending is to get a terrible ability score up to 'average,' to get an average score up to above average (which is more expensive) improving a proficiency or gaining extra ones, creating a unique ability for your character,etc.
Non-XP rewards can be a form of experience banking. If, for example, a mage is spending every waking moment asking the high-class knight in the party about his adventures, the DM might want to give the mage and the knight some extra experience in the hopes that they'd bank it, or the DM might decide that after weeks of learning, the mage has picked up the proficiency "etiquette" (or maybe bonus points in it) and the knight has picked up storytelling.
Non-XP rewards I have given have included free proficiencies, a special attack, extra renown for a bard, followers, friends in high places and a level of training in a different class. Usually the non-XP reward is at a 'discount.' (For example, a proficiency costs 3000xp on the banking chart, but I once gave everyone in the party 5000xp and one character a choice of 5000xp or 4000xp and a free proficiency in a skill his character was using constantly.) I've also allowed class combinations not typically allowed as a result of this.
The logic behind it is that if a character is really focused on something, he's getting better at it faster than 'normal.' So a mage who always casts the same two spells might become the expert in those two spells. The DM can, for example, start to add abilities to that spell (the Diviner who always cast "arcane bolt," for example, started to regularly get a flash of information about the target when it hit, creating a signature spell form of arcane bolt that had Divination benefits, the BOOM mage who **always** cast wall of iron started being able to make much more complex walls of iron because he knew the spell backwards and forwards) or the DM might even say "you know, you do Magic Missile so much, I'm going to say you've memorized it permanently, and can cast it once a day for free. More than that and you're going to have to lose first level spell slots to do it, though."
These sorts of non-XP rewards can create complex characters that aren't cookie cutters of each other. It can make it hard to move a character from DM to DM, but if you're staying in the same campaign, it's very useful.
Sometimes, these non-XP rewards become so common they end up game constants. In my own world, several paladins in a row kept gaining the ability to give off visible holy light in their goodness. After a few years, it became default that paladins could do this sparkle as an ability, and we added it to the host of the paladin's skills. (It is enough light to see by, but not enough to do damage, unless the 'holy aura' would do damage normally.)
It does require, IMO, a bit more finesse as a DM to pull off this. I sort of consider giving XP to be the default, easy way to reward a party, allowing XP banking to be the slightly harder variation and XP+Banking+non-XP rewards to be the most difficult.
Non-XP rewards can be very important to clerics. I basically assume a cleric being extra clericy is going to get a reward from the top.