Firstly let's explain the reason why people don't like XP debt.
1) They are just not used to being punished with a risk/reward system. So being punished for things that happen doesn't feel good as many people are used to casually grinding through content to get rewards. Simply I think not being used to it as it hasn't been a standard part of of MMOs is part of the reason.
2) People can potentially get abused and put into a unfair situations where they get PKed over and over and they have no control over it and the amount of XP debt needed to grind back is demotivating.
3) Overall people like to feel they are progressing with their character and if XP debt over time stops their progression too much they will quit the game.
During the phase of levelling to max level skill/guilds/community will play a part in how much XP debt you incurr getting to max level which can make it a bit more interesting than just pure grinding. However for me I believe XP debt can be most useful when at max level. This is because in some ways it adds more content to the game because in most MMOs you level up/quest and then you are max level done. However if you incurr XP debt at max level then it's still important to do quests/gain XP every now and again which adds another element of replayable content. You maybe at XP debt at max level but then see some quests at a node that just levelled up, so now you have a reason to do those quests to get xp. Also this XP feeds into levelling the node it that area. It creates more content to do.
For me what's lacking in most MMOs is endgame as it's mostly just grinding instanced raids/dungeons for items/cosmetics which gets a bit repetitive and stale in my experience. So my hope is this XP debt works as an XP sink which you replenish with another form of enjoyable gameplay. The way I see why MMORPGs can be fun is the mixture of content they can give you even at end game. xp sink so you still do levelling(open world PvE)/quests at max level, crafting, open world pvp, node/castle/siege pvp/ arena pvp, dungeons, raids, economy, exploration.
thoughts?