Interesting. I was going to write a follow up post saying that instead of doing XP bonuses, the reward should be something along the lines of "reputation points," which players can use or turn in for some sort of reward. That seems like it would make more sense than XP for the following reasons:
1. It's more thematic. It makes sense that if a mercenary spent a lot of time building his reputation, that he would get some sort of special reputation based bonus (like access to contacts or bonuses on diplomacy checks), but it makes less sense that his spells would get more powerful.
2. It's more flavorful. XP awards just go into the same pool along with the regular XP awards, but if you pull them out into a new system, then it makes them more unique. Also if you are trying to compelte for who gets the awards, it makes that part a little easier to keep track of.
3. Different groups handle XP very differently. For example, some of the groups I've been in has the DM hand out XP at the end of every session, some at the end of every encounter, some don't bother with XP and the DM just tells you when you level, some have more "attendance bonuses" or "roleplaying bonuses" than others, etc. (I don't think I've ever been in a group that does it exactly the way the DMG describes.) Tying the bonus reward system to the XP award system makes it less compatible with all these different XP systems.
That seems kind of like what you're suggesting with the 500 xp = 1 talent system. But maybe you could have a separate list of rewards just for the bonus XP.
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Also, just some clarifications:
Brutality: My question about "turning off" the Brutality was not, if I Brutalize an opponent can I erase the evidence. It was, when I kill an opponent, can I choose not to Brutalize him if I want to stay hidden?
Also the way it's described, the 10% XP bonus seems to me to be more like a (10%/n) XP bonus, where n is the number of characters in the party. For example, suppose that there are five characters in the party, and five 100 XP opponents. Each character kills one opponent.
Without Brutality - Each character gets a 20 XP share from each monster, for 100 XP total.
With Brutality - Each character gets a 22 XP share (20 + 10% Brutality bonus) from the monster he killed, and 20 XP from all the other monsters (since he wasn't the one that killed the other monsters, he doesn't get the Brutality bonus) for 102 XP total.
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Experience Awards: I see what you're saying, but that wasn't the impression I got from the article. The article explicitly says that you get the XP rewards for civilians you kill "even off the job," which implies to me that it's regardless of if the target is related to the job. Maybe you should clarify it by saying only if killing the guy helps advance the mission in some way or makes it more likely to be undetected.
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Ruthless Appearance: So you mean the Charisma bonus does apply to attack and damage rolls for Charisma based powers? If I were a high paragon tier warlock merc, I certainly would "go all Diego Montoya" (I assume you mean Inigo Montoya, as in "I am Inigo Montoya, you killed my father, prepare to die") on every enemy if it would give me +4 to attack and damage rolls for the entire encounter. This also makes charisma based characters much more powerful than characters with other primary ability stats.