Rewards (XP, gold, loot), that's all carrots and sticks. I make a point to not be an addict to that stuff, but I also realize that there are players who are addicted to that stuff and that's bad. More generally, I don't really appreciate the fact that there are carrots and sticks in games, GW2 is no exception. I appreciate it's not as bad in GW2 as in other MMORPGs, but that makes it less evil.
I'm also making a distinction between "grind (noun)" and "grindy (adjective)". It's part of my "getting to understand what makes games what they are" series. Grind (noun) can be seen as content, whereas grindy (adjective) is the boredom associated with the obligation to complete a given amount of content (that you might want to skip) to progress.
The problem with levelling is that it pretty much confuses content with rewards. But it also takes content and makes it so you can pick & choose which parts you want to do, and which parts you want to skip. See, if each heart was a closed room with foes and you had to kill all the foes to open the door to the next room (as is often the case in fps games), nobody would call that grindy. But because you have levelling and open areas and all that, now we're all "but I don't wanna kill stuff" which is absurd given we spend so much on combat games.
I do want to kill stuff. It's also absurd to think the endgame foes are fundamentally more interesting than midgame foes, unless that is, that the endgame is fundamentally different from midgame which it shouldn't be.
Yes, there are people who get bored of content and do it anyway because they need the XP. That, I find terrible. But then there are also people who will skip content as much as they can regardless to get to endgame, and then complain the game has nothing to offer, and that's also bad. And linear games feel linear which is also bad. The point of levelling is to offer open-world, but also to pace the player, to make sure they do get to play the content.
Hearts are content. They may be somewhat cut & paste content, but it is nevertheless quests with mobs and scenery and stuff that needs to be done for some lore-based reason however ridiculous it might be. If you removed gold and XP entirely from the game, I'd still do a bunch of hearts just because they're something to do and I enjoy playing the game. I'd likewise still do jumping puzzles for fun, or sPvP for fun. The fact that some content include rewards does not make it any less content... but it does make it somewhat more obligatory.