There's not much that bothers me. It's an original product, and if you don't like it, move on to another hack. Honestly, I am more bothered, as a romhacker, by followers that feel the need to tell me I am doing it wrong-- I have hacks without fakémon, but they chastise me for having one hack with fakémon, for example. It's beneficial when someone contributes, not so much when they say "you are doing it wrong." It's kinda like a backseat driver.
That said, there are a few places I draw the line.
1: Spelling and grammar. Regional dialects are alright (Gamefreak even uses "rural" dialects for some characters, and you can't really hold someone at fault for British/American English (an before "h"s, for example, or colour vs color), when the former is a purer version of the language and the latter is quickly becoming the world standard. Nor is an error here or there intolerable, if the quality of the hack is sufficient (such as, say, AshGrey.) However, if the creator clearly has little mastery of english or isn't trying, or if there's not really anything else there to make up for it, it's the most key part of an hack and can really turn it into something along the lines of a bad Twilight fanfiction in terms of quality.
2: Mapping errors-- Blocks that don't mesh, or walls you can walk through. I can tolerate "unnatural" maps (like Quartz's remixes of the cities of Hoenn), since the natural thing is better but not essential, but having constant visual glitches is frustrating and breaks the suspension of belief necessary for role-playing games.
3: BAD fakémon. I mean, like Quartz, which has both terrible designs and, in spite of some early posts ITT, bad spriting-- The lines are straight, the proportions wonky, the parts misaligned and the shading of poorer quality than the earliest graphical RPG's. On the other hand, good fakémon provide entertainment for me and make an hack original and enjoyable; Sadly, we don't see many of these, and most of the fakémon hacks we get are rushed and low-effort/low-skill endeavours that end up being something that could have been much better with canon 'mons.
4: Game-breaking glitches. Honestly, I've played very few completed hacks that actually have these, since most people use VBA, but as someone that uses no$gba (runs faster, is more flexible) outside of palette editing, it's frustrating when, for example, it fails the header checksum in no$ and thus is unplayable without a different emulator; And, of course, broken scripts do pop up occasionally, and should be kept out of completed hacks.
So, basically, glitches and things that are plainly and undeniably ugly; Everything else is a subjective matter of personal preference, and while one person may not like, say, Touhoumon, I enjoy it greatly. And, of course, there are things like Quartz that are so bad they're good (though I applaud Baro on keeping it technically glitch-free).