I wouldn't say "hate", that word is too harsh. And it also makes you sound arrogant, like if youre better than someone else. Remember, everyone starts/started at a low level. If you dont like the art, or the writing, dont read it. Topic done.
About the lack of updates... well, thats the one that really kills a comic, a fiction or ANY series.
After all, I would agree to what Nereza said above - mixture makes it. The art does not need to be perfect if the story is good. On the other hand, if the story isn't perfect, maybe there can be some good art in it that makes it up.

Both bad art, and bad writing are big no-no's for me.

i don't mind the update i know art takes time to do and people have life's although no matter how good the art is if the righting is bad and allover the place it makes it almost unenjoyably as a comic at the same time i read some comics with shitty art that had great story's some time if the art is lacking too much it makes it unbearable to look at so a a little of a and b personalty i like different sorts of stylization for artwork so to me the story and plot matters a bit more then the artwork but the artwork style and skill level is still important to me when reading web comics ...

I can't vote for more than 1?

While bad writing is a pretty big one for me, I can forgive it sometimes. If it's just dialogue or predictable character interactions while having a great world, plot or story then I can forgive it.

But lack of updates on any comic kills me. Especially when it gets close to some major plot point. Then they move, something happens, and they forget about it even if the art is good but the writing sucks (I'm looking at you Chugworth)

I'd have to say none of these. I don't read comics and then judge the artist in such a manor. Its a learning experience for them and they have the courage to place there art and stories to the public to read hoping not only for positive feed back, but maybe even critiques so that they can strength there weak points.

I'd say mainly bad writing. I can understand a lack of updates so long as they don't keep saying "oh it'll be soon" and bad art seems to the thing people are better at improving on. Take the example of Gunnerkrigg Court: when it started, the art was atrocious. But it got better and is now pretty popular.

Bad writing as in if the storyline gets to jumbled and complicated and the author does not stick to the writing style.

And also lack of update. I mean, once a month is okay, but spontaneous updates at random monthly intervals is a bit too much for me.

If both are bad, which is usually the case, it can be a big turn off, but I will tend to give story more of a chance before the art. Sometimes art styles will grow on you or are "bad" on purpose.

Well, most of the amateur comics i've seen had interesting concepts, good writing and good enough art (or even some with great art), so the first problem is the artist can't be drawing as much as a professional would, so updates take looong and they leave you hanging.