@smooth, as you are here discussing about the topic, I really would like to know your opinion about a reward curve which starts non-linear but ends linear?
I think that's a pretty good idea. Another way to describe that is a penalty, tax, or some similar terms, on the low end which discourages milking of rewards via high volume spam. I've written in support of essentially the same idea and Steemit even proposed it once. Apparently there are some technical obstacles to implementing at least some versions of it though.
I think also implementing diminishing returns when upvoting the same account
I don't support that because it encourages creating large stables of round robin accounts to evade the repeat voting rule and/or the four post rule. That doesn't help anything and in fact adds more costs onto the system. The four post rule (and maybe the repeat voting rule) also disadvantages many plausible blockchain usages that are different from long-form blogging. Finally, comments are rewarded just as posts are. Applied to comments, four is much too low, so your rule would either encourage reward milkers to use more comments or would discourage comments. Neither is desirable.
See my comments above. As much as we would like people to behave in the desired manner, we can't wish that into existence. All system rules must consider and be evaluated in light of the full range of possible human behavior including evasion or 'misuse'.
Thanks for your input/opinions, I really appreciate it.