So, with that in mind, I'll tell you a few things that will catch the attention of people looking for writing, and for writers. I'm not the biggest expert in the field by any stretch of the imagination but I can tell you what floats my boat and what makes me want to beat my head in.
The immortal enemy, the historical foe, the... ah, the hell with it. It's not going to go away, no matter how much you want it to. You can kick and scream, but it's one of the very first things that we look at when we look at your work. If I read a story and the grammar is wretched, I will click to the next page and not bother reading the rest of it.
By wretched, I mean 128 words without punctuation. I've seen that, and not in poetry. Your grammar doesn't have to be perfect; we all make mistakes. I make them quite frequently, myself, and I am not afraid to admit that. I'm not perfect. And the number of mistakes I make goes up exponentially depending on how much sleep I've had. But at the same time, I punctuate things and do my best to make sure that it's at least readable. The primary places I will look for grammar are: appropriate capitalization, basic punctuation (periods, commas, apostrophes, etc.), and the proper Your (you're), Their (there, they're) and other such common mistakes.
The occasional slip up won't put your head on the chopping block, but if your entire story is grammatical error after grammatical error then it is far too painful to bother reading. I have had stories where my edits to the grammar and spelling would have been longer than the story itself. I don't get paid by the edit, I get paid by the publication and as such I'm less inclined to spend four hours editing something that may not make print if I don't get paid.
When I'm reading websites that are not formal submissions I'm far more likely to be a bit more lenient than I am when reading something that someone is officially sending me - since most of these people aren't trying to get those stories published as is, they're just getting their writing out there. But the people that have good grammar definitely attract far more attention.
Typoes are inevitable when writing a first draft and, again, if I'm reading things on other websites etc. I'm likely to be forgiving, but good spelling definitely gets you far more attention than not, that's for certain. Make sure that you're not making common mistakes and are using the proper words as well (refer to the last segment regarding "there", "their" and "they're"). When posting things anywhere it doesn't take long to run a quick spell check, Firefox and many other web browsers have one added on that will note you if there's a word that it doesn't know. Just keep an eye on it and you will go a long ways. One of my pet peeves is people that are writers spelling things phonetically. It saddens me that there are so many people that fancy themselves writers that can't be bothered to pick up a dictionary or double check their writing.
If your story doesn't grab me in the first few lines I'm probably not going to swallow all of it. You want to make sure that you sink your claws into the reader in the first few sentences and don't let go. Now, by "sinking your claws in" I don't necessarily mean that you have to start "in media res" but the description should be gripping and draw the reader in quickly. Whether it is an absolutely lush description of an alien world or starting with a bang - literally - like a murder or some kind of other scene that leaves the reader wanting to know more, your job is to attract them in the first few lines.
Once you've attracted them to keep reading, your job doesn't stop. You need to keep their attention. One of the biggest problems I've had with many of the writers I've read is that they start out interesting and absolutely kill the middle of the story. I'm not going to pretend that I am not guilty of that folly on occasion, but it's something we all have to watch out for. However, the biggest and most important thing is to keep the reader's interest and that isn't necessarily an easy thing to do. Contemporary audiences have video games, movies, music constantly, and all manner of entertainments to divert them - you have to compete with all of those things and make your book so gripping they won't want to put it down. Even for video games.
I want stories that are realistic. If you haven't done the research (or haven't faked it well enough to pretend you did) I tend to pick up on that quickly. Writing stories without doing research is a huge mistake, it's like how in Twilight the vampires come from an Island on the west cost of Brazil. If you know anything about geography, that'll make your head explode.
The old adage "Write what you know" absolutely applies, but I think there is an addendum that needs to be tacked on the end of it, "Write what you know, research what you don't." This is especially true of specialized fields, like policework or medicine. If you want anyone to believe your writing, you have to know the facts as best you are able. I don't suggest that you get your degree in medicine before you write about an injury - there are a number of fantastic reference books out there you can buy (Body Trauma is one of them) that will fill in information specifically for writers.
I read a story once where a man had a torn aorta from a bullet wound. Not only did he survive the 30 minute wait before the medics got there, he was conscious and talking the whole time and not being administered any manner of first aid. Now, without a degree in medicine, I tilted my head and said "that doesn't sound right". The aorta is one of the main blood vessels surrounding the heart and with that being torn, blood would be pouring into the chest cavity. You would bleed out and drown yourself at the same time, it would be quite the end of you in a very swift manner.
It's things like that that tend to really, really turn me off of a story, almost as much as poor grammar (and that's among my top pet peeves). Spending 10 minutes on Google will give you enough information to write at least a little about almost anything, if you know how to punch in key words, and most of the time that's all you'll need. In addition to that, if you're writing about a real city, it's a huge help to stroll down the streets via Googlemaps. Even if you can't drive there and wander around the streets of London yourself, you can Google most anything and find it. The information age has really made available all those details that previously we had to spend hours at the library (or witness personally) to cover. However, the revolution of the internet has offered us the ability to research anything in just a few keystrokes, removing the excuse that you don't know the information.