To check the balance, simply play a noble level and then play a deity level and watch how different the education plays out and then extrapolate out the ramp up from noble to deity and you've got a fair judgement of the influence it has on play at each general level. The more education pop soaks up, the less the buildings are sufficient and the more units are necessary or population becomes best to keep under manageable levels. The population management of keeping control of education is actually one of the first times I've seen Civ suggest the truth... that there really can be too many people in one place for it to be beneficial.
Hm... This makes me think, maybe there could be a (very cheap) building which would eat all your excess food and convert it into


You could do this but its already pretty simple at 1. I fear anything less than noble is just too easy already.
I was certainly not intending to put 1 (even 2 education or crime feels easy in my experience), but something already currently challenging without being outright unavoidable - keeping in mind that buildings can be tweaked from there if necessary. 4 for each would be ideal actually, since the decay is 4% it would be very easy to figure out where your property is headed: baseline is 100 (or -100) per pop, each +4 properties adds or removes 100. Or even slightly increasing decay to 5%, putting base property/pop at 5 so that there would be buildings with multiple of 5 properties (like currently for crime) that would each change the limit by 100 for each +/- 5 properties: much easier to figure out than the current system.
From there it'd also be easy to figure out how many buildings (and at what cost) are necessary to maintain a given city (or more accurately, control how large the cities can be at a given point in the game), as well as what the threshold would mean (100 is what you'll get in a city size 1 if you don't add any property from buildings/units or a city size 2 with -4 properties from buildings/units, 200 is what you'll get in a city size 2 with no property from buildings/units, etc.; 1000 is normally unreachable by a city under size 10 and very unlikely for even a city at 10 pop or slightly above, etc.).
Actually, properties (disease comes to mind) could even be use to control city growth through a feedback loop - if your city is unusually large, it'll be crippled by diseases that act as -X%


I can see there being a point to this. But then, I also see a point to figuring out how to eventually harmonize our core with nightmare and eliminate the option since ppl seem to feel there are too many options.
To rephrase what I think you're saying in that last segment, you're suggesting "Let nighmare mode begin to mean 'a game where the difficulty levels include increasing the difficulty from properties where the normal game would generally not.'
Properties, but maybe also other things that specifically affect the player, like maintenance cost, goody huts results, free base happiness or health... Nightmare mode would then mean, "it's not just the AI that is more competitive, but the player would also have to adapt to harsher game mechanics".
Though that's not my favourite option among the two, because we'd have an additional game option, and because either "nightmare mode" would be too harsh or it'd be a good enough challenge so that its absence would feel like removing a feature (exactly what you said about Noble with properties not really needing maintenance).