Currently sub categories are not very rewarding because of the exponential value of the vote.
take a category that has in average 10 votes and one that has 20. The one with 10 gets 10 times 10 = 100 payout, the one with 20 gets 20 times 20 = 400 payout. Double the voting power means four time more payout. Therefore all try to fish in the know categories, which encourages hashtag abuse.

I would suggest to make the payout linear, or at least more linear. In away its your money that is redistributed with up to 10% each year (currently even lot more with round about 0,1% per day) So it should be your choice. I know the exponential payout has two reason, one is to dis-encourage self-votes, second to have some high ranked payouts like a lottery that can attract more useres. Self-payments could be solved through transparency. If someone becomes know as all the time self-voting he can be warned (yellow card) and then down-voted - flagged (red-card)if needed.

To the second: Currently posts are not only rewarded exponentially, but also they get a lot more votes if they become visible on the trending page. So even with linear vote-counting, they are rewarded exponentially, because they attract more voters through being visible.
In away it would be fair if the posts which are displayed on the main site are drawn in a lottery like style. Every time you reload the page the displayed post are drawn though a lottery, the more votes they have the higher the chance to be selected. This would give all participants a fair chance to be listed at the top and therefore attract more votes.

That was just my thoughts to improve steemid for a hopefully better experience for all :)