Originality.
Look, just about anyone can edit a game and call it a "hack" (heck, I've done a few edits myself, but I don't consider them to be a hack, just an improvement or modification), and many people these days don't really put much time and effort into a game in order for it to be ORIGINAL.
Now, don't get me wrong, many hacks (despite being a rip-off or clone of some sort) tend to follow suite, sticking to the generic "I'm gonna become a pokemon master" idea or "I'm going to save the world from Team Nobody-Cares-What-Their-Name-Is", but in the end, the most amazing hacks are the ones which don't follow the outline that Pokemon is.
When a pokemon hack is more than just playing Pokemon, that is when I am genuinely intrigued. As I said before, there are so many great hacks which are essentially copies of the original game but with changed graphics/sounds/characters/etc, and they are indeed outstanding to see... but in the end, they're not really any different than the original game.
For me personally, the hacks that I truly enjoy are the FAKEMON hacks, these ones please me the most, and this is simply because they are just 1 more pokemon collection for me to gather. I mean heck, I've already caught all 721 Pokemon (+Mega Evolutions and a few of the newest ones in the later games), but to have even more Pokemon to track down and add to my family/team/party is really something special to me.
But sadly, there are many FAKEMON hacks which are not in English or are so poorly done, they can't even be played entirely (and some which are not complete yet). So in the end, I get what I can, and slowly add them to my roster.
Now of course, 99.9% of these are definitely a CLONE of the original game, just with different Pokemon to capture. But hey, that's the beauty of it.
Now once again, I'm not much of a fan of clones and there are only a few exceptions to the cloning rule, but more importantly, the games that truly and I mean TRULY capture my eye and keep me playing are the ones which don't pursue the usually "I want to be the very best" or the same old slogan of "I will become a Pokemon Master" or "I will be the Elite 4!" or "I will conquer the WHATEVER-THE-NAME-IS League".
The games that really keep me addicted are the ones which don't play like regular Pokemon, the ones that have something about them which isn't the same old song and dance.
I'm still waiting for the day where there is a Pokemon game that doesn't make you fight trainers that see you, doesn't expect you to become a Pokemon Master, doesn't expect you to become a Champion, doesn't want you to save the world from some disaster caused by Team NOBODY-GIVES-A-****.
I want to hopefully live to see the day when Pokemon expands out from the usual crap that it keeps rehashing over and over and over and over again and does something new. Heck, this is why I still play Pokemon Snap, it's different. I want a pokemon game where you can just be a NPC, live a calm life, raise your pokemon, die with your pokemon, just... be a normal human being.
And of course, I also want for someone to someday make a pokemon hack where you can simply just battle non-stop, like that Pokemon Battle Factory hack, that was something. It would be amazing to see a GBA version of Pokemon Stadium or something like that, or even a hack which is nothing but an endless randomized Battle Frontier, that'd be amazing too.