Re: Exp in L85 areas
For zones like the Pits that are L85, do you get more experience from the monsters than you would if you killed them in the Black Marsh? Ie, is their base experience scaled up to L85, or do you just not suffer as much of an exp penalty against them?
When determining the experience of a monster, the games does use a modifier which is dependent on the monsters mlvl. Basically it's (BaseExp * ExpMod) / 100.
For example, the experience modifier for lvl 83 monsters on hell (ladder) is 48477, while the experience modifier for lvl 85 monsters on hell (ladder) is 51195. A skeletons base experience on hell is 85, so a mlvl 83 skeleton would give 41205 experience ((85*48477)/100), while a mlvl 85 skeleton would give 43515 experience ((85*51195)/100).
So basically yes, monsters that have a higher mlvl give more experience, although the rule could only be said authoritatively if we're talking about the same base experience, which brings me to point 2:
And why is CS such a better exp run than other L85 areas like River of Flame? Monster density? Do the superuniques there give markedly better exp? How come people run Chaos so much but not other L85 zones?
Every monster has a different base experience, which is then amplified by the exp modifier based on the mlvl. For example, as mentioned above skeletons' have a base experience of 85, but balrogs (megademons) have a base experience of 130. The storm casters have a base experience of 135, and oblivion knights have a base exp of 210. Because the base experience is higher but the experience modifier stays the same for lvl 85 monsters, the overall experience will be higher.
Because of the differences in base experience, it's not 100% correct to always say that a higher mlvl monster will have more experience than a lower one. For example, a lvl 85 skeleton on hell ladder gives 43515 experience as pointed out above, but a lvl 83 balrog would give ((130*48477)/100) = 63020 experience, which is more despite it being a lower mlvl.
And finally, yes the clvl --> mlvl penalties would also play a factor.