I think you should reply back to someone who only says "hi"...it's possible that he didn't think that you would reply, that's why he didn't bother sending a long message (my current boyfriend did this). It's what they say after the "hi" that you should pay attention to. Also, it's good to look on their profile as well, see what they are about, where they live, what their interests are etc.
Usually guys do like to meet up within a day or two, for them it's an easier way of breaking the ice, seeing if you have things in common, and most importantly, finding out if you are real. I wouldn't meet with anyone who I didn't talk to for at least a week, so I just used to say something like "well I'd prefer to chat on here for a bit longer" and see what they say. Most of them wont mind, but if they do, then they are not worth your time and are looking for an easy hook up....watch out for these guys!
Also, don't take to heart those guys that get angry at you, you're probably the 5th woman that day to say no thanks, so they are feeling quite low in their self esteem anyway....if they start being rude online, block them, you don't need to listen to a stranger talk about how rude you were and call you names, that's not what you are online for.
PM me if you need more advice, I have loads!