1. People will form a low impression of you.
2. People will think you are some sort of noob. Especially in online games
3. People will not understand what you are trying to tell. Now this is bad.
4. You will not understand what websites are talking about.
5. You will not get enough information you need to carry out your tasks.
6. You cannot carry out your tasks, like posting stuff on the Internet in English!
7. You will misunderstand what you read and think that you understood the correct thing!!
But the Internet is full of websites that teach English, and try to help you improve your English. But from what I found, most professional sites teaching English are already full of complex English. I mean... they use high-level English sentences in teaching English. So how is the mediocre English-user going to learn and understand? And even worse, some websites teaching English contain grammatical errors in their text. Oh My God.
I believe the best way for one to improve English is to actually start from a lower level. Maybe if they can find a site that teaches English in their native tongue and slowly build up. Or at least a site that is giving explanations using simpler English, like this English Language Blog.
That way, those who are poor in English can actually understand what is being said on the site teaching them. No use learning from a site that is teaching you using the level of language beyond your comprehension, right?