Mistakes aren't necessarily a bad thing, but if you can learn from other people's mistakes it can save you from having to deal with them yourself. When it comes to web hosting, there are basically two kinds of mistakes - technical and general business.
Technical mistakes usually come up because of a misunderstanding of the internet and how it actually works. The first mistake many people make when creating a website is to cram as much information, photographs, images, etc. on each page as possible.
This makes the site take longer to download, leading to many visitors just moving on and never actually looking at the site. It also makes it more difficult to find what they're looking for if the page is unorganized.
Another common error is creating a web site that isn't search engine friendly. If the search engines can't determine what your site is about, they aren't going to be able to send you people who are searching for what you offer.
A mistake that many people make when starting out is to choose a host solely based on price. This is obviously an important factor, especially in the beginning, but if you choose the cheapest host you may be limited in your scalability as the website grows in popularity and traffic.
On the business side of things, the most common mistake is trying to be everything to everyone. You should have a plan for your website - a purpose for its existence - and build according to that plan.
Choose your target market and stick with it. Advertise in places they would see. Market in ways that would be of interest to them. Resist the urge to branch out into other areas just because something catches your eye. A site that tries to do everything usually ends up achieving nothing.
Another common business mistake is following the competition's lead. You'll obviously want to keep an eye on them to see what they're up to, but if you copy everything they do you'll always be one step behind.
Use your creativity and come up with unique ways to stand out from the rest of the websites in your market.