The bitcoin hard fork has come along because some people believe bitcoin needs to be able to allow more than it's current 7 transactions per second.
The steemit hard fork is different. It didn't need to happen to allow for growth, it's just a few tweaks the witnesses agree would be improvements.
There will probably be fewer hard forks as steemit gets closer to leaving beta and launching officially.
Marginal utility is the proper term for the concept I touched on.
A third car might not provide enough 'utility' to justify it's cost, putting it on the 'margin'.
The law of diminishing marginal utility just means you value each possible unit less than the ones you own, since they're already meeting your higher priorities.
(Driving to work, picking the kids up, running errands etc)
Linear just means its a straight line, like a ramp. Each dollar you have in the bank increases your buying power by one dollar.
Exponential is more of a curve, like a skateboard ramp.
Each additional digit in your PIN increases your security x 10