Reading through the various posts I've seen here regarding etiquette, it's hard for me to put aside the "downvote" vs. "flag" issue because, to me, it's the crux of the confusion as more and more people describe what the flag is for (preventing abuse). I get that a minority of people use other interfaces for the Steem blockchain, but I think it's safe to argue most use Steemit.com which does make a distinction between a downvote and a flag.
I guess, for me, it's hard to agree with all abuse being subjective (I'm coming from Sam Harris' Moral Landscape perspective), because some things the community, as a strong majority, does come to agreement on, such as frivolously downvoting being a detrimental activity for the network.
I agree with you, excessive upvoting can be seen as a form of abuse, especially by the very small minority which currently have such a huge influence over total payouts. Currently the interface provides no simple mechanism for: "I like the post, but feel the payout is excessive, so I'm going to vote against it using my own influence to bring the payout down, thus leaving more payouts for others." If the flag had a "reason" option which included some text (it could even just auto-post a comment to that effect if we didn't want to change the blockchain structure for the new meta data about the flag), then I think it would solve so much confusion here. It would even still follow the "abuse prevention" intent a flag implies.