I should say one thing here:
I don't want to see anyone say "I hate when a hack isn't finished" or "I hate when a hack isn't in English". They're... just not fair to point out, seeing as they really have nothing to do with the quality of a hack/skill of the hacker (at least the latter statement doesn't).
There are a few things that actually annoy me about hacking, though, some of which you've mentioned.
1. I hate when I spend a lot of time inserting new tiles, and eventually get really tired of them. It's my own problem, but... still very annoying.
2. I'm also a bit annoyed by the repetition necessary when testing scripts and stuff; although when I finally get what I want to happen, that makes it worth the while.
When PLAYING hacks, there are some things I find annoying:
1. As stated above, when there's an alpha up, but it literally covers the first few maps/events. Just hold OUT and wait until you can give us something we can play for a little while- there's no need to try and keep people hooked by giving them the tiniest of tasters- people often post threads (that I can't approve) asking for a list of full hacks. People prefer having more to play, even if that means they have to wait a bit longer.
2. Of course, general errors. I mean a hack doesn't have to be perfect, but it does have to be presentable. So tile errors and stuff shouldn't be present- and EVERYBODY can come to recognise tile errors if you stare at A-Map long enough. Generally, I think people should step back and spend longer learning the basics before jumping right in and posting a thread for their hack.
3. This last one's a personal thing, one that only moderator's of this section have dealt with- when I get a thread that has no screens or anything, but it offers a download. I have to test the download to see if the hack actually has enough progress to warrant a thread. Usually, it's FireRed with the first few maps edited- most often the paths in Pallet Town are changed to the Tall Grass, the houses are replaced with caves, etc. I remember more than one hack (from different people) where you end up going to fight Moltres, Articuno, Zapdos, Lugia and Ho-Oh as soon as you step outside the door. I guess this returns to my previous pet peeve- people spending far too little time on learning about hacking, finding out how to do the generic 'awesome' things, doing said 'awesome' things, and immediately posting it.