If any of these resonate with you, avoid them...

I can do this for free. [/b]You cannot build a business online for free. Get over it.
[b]Build it, and they will come. You cannot just put a website up, and expect to start raking in the profits.
Decorate it, "Trick it out", not for websites...this might work for brick and mortar stores, but online you have to keep it to reasonable level.
Lacking a Unique Selling Proposition. You need to generate a real presence online...and you have to be unique.
"I don't have to work hard to succeed online." Pfui. Sometimes harder.
Earn money in your sleep, 24/7/365, etc... Yes, you can, but your business requires attention.
"I already know what to do, I don't need a game plan, plans are for newbies." You might as well count yourself out now, turn in the keys and go home.
Not getting to the point. [/b]People have so many other distractions nowadays, you don't have time to go into theory, just get to the point.
[b]Writing for the offline audience is the same as writing for the online audience. [/b]Wrong answer, thank you for playing the game.
[b]Failing to remember you are dealing with people online. It is so easy to do online, and the impersonal nature of the media can help you lose business as fast as you can gain it.
Stopping at the first sale with a customer. Big error, this is only your first contact with a person who said "I trust you"...what are you going to do next?
Give them more reasons to show up again and again. It doesn't stop at the first visit, what are you going to do to keep 'em coming back?
Terrible follow up system. Are you following up with someone who gave their email address and name to you by sending endless offers?
"I have 10 million hits and no sales...how is that possible?" Traffic isn't the key, targeted (quality) traffic is.
Don't make them think. Getting your visitors the information they are seeking as quickly and easily as possible should be a primary objective.
Lack of focus on your business. [/b]There is more ways to make money on the internet than you could write a book on...but you need to pick one and focus on it until it develops profits consistently.
[b]Failing to re-invest in your online business. You always need to focus a certain percentage of revenue back into the business.
[b]Failing to properly track and monitor your online business. [/b]It is so easy to do this online, yet many don't even try.

Well, there they are, 18 of the most critical mistakes you can make that will kill your online business
Source;http://voices.yahoo.com/18-mistakes-kill-online-business-890663.html

nice expecially the follow up thing, that shows ur experienced in life.