I like the idea behind it more than the execution. Agree that combat XP in this system should be based on the overall encounter rather than the "number of creatures defeated", and overall I don't either the math or the scaling work out very well.
The "XP for treasure" concept holds very little nostalgic value for me and I don't miss it from the game; for any kind of discovery to be valuable enough for me to warrant XP gains I'd stick solely with the scale presented under Locations, so I'd probably just lump Items/Location under the same system. A PC shouldn't get XP for finding a fancy magic sword, not unless it has some other extrinsic value, such as it being an ancient relic of a long-dead king that could be used to bolster an allied prince's claim to the throne.
I'd also make the case that there's often more to social interactions than "swaying an NPC". That's fairly vague and broad enough to cover a lot of social interactions, but certainly not all of them. Also, the level of influence a particular NPC has is not necessarily correlated to the difficulty it takes to sway them, or the significance swaying them could have for any given adventure or plot.
I've played enough video game SRPGs to be able to wrap my head around the whole "100 XP to level up then start over again", I'm just happy the XP carries over; most of those games immediately drop you back to 0 regardless of how much left over XP you earned! This system currently though feels not just like a treadmill but like that treadmill from the Jetson's intro (just realized that's my second classic Hanna-Barbara reference on this board today). Unless the math is tweaked this feels like it's going to be used in a heavily accelerated campaign.
I've been using milestone leveling since making the switch to 3.0, and the one time I tried switching back to XP (in 3.5) I was not really a huge fan of it. This has advantages over the original model of highlighting the significance of the exploration and social pillars but I'm not certain, ultimately, who this is for, from a player aesthetic side. Players who want to discover new things or engage in social conversations are already doing those, and being able to engage in those actions often serve as their rewards, and the players who are mostly there to kick down doors and kill orcs are going to now feel obligated to engage in the game in ways they probably don't enjoy in the first place (they may learn to enjoy, but most people are pretty good at knowing what they inherently like, even if it may sometimes be difficult to explain it).
Meanwhile Milestone scratches all of those itches without piling on incremental rewards that force players to engage with game aesthetics that they'd prefer not to have to deal with and even less record-keeping.
Again, I like the idea of creating an XP system that incorporates all three pillars fairly evenly. This attempt misses the mark for me in a lot of ways, and ultimately I think I'm not a fan of XP systems at all.
Left that method back in the 3.5 days. Just use milestone leveling, its way simpler and prevents the ever-constant 'is that worth xp?' and killing creatures just for the XP. Been in over 6 groups using that method and it works wonderfully.
