The rewards, of course :)
You're spot on - we need to curate responsibly, as a community. We should stop mindlessly voting on just our friends and ourselves, and on the trending posts. Besides, that's a sure fire way to kill off all your curation rewards.
The community must be educated to vote on great, undiscovered content and promote them. There's a significant incentive in the curation rewards system that favours voting on such content versus posts that are already trending.
Despite a largely positive HF19, I'm afraid after HF19 I see a lot of engaged curators with large SP holdings (or delegated) give out 10%-25% for others, while saving 100% for themselves and their friends/colluders. That's not the desired behaviour of the voting power rate change. This should be called out as questionable behaviour.
RE: Which came first, the Engagement or the Reward?