1. Too much text
People like pictures, plain and simple. The way users interact with websites, and the world in general, is becoming increasingly visual and there is research to back it up. Visual cues increase engagement. When designing, think about simple graphics that can relay the same messages as cluttered boxes of text. Users will be more engaged with your site and your design will be cleaner and more appealing.
2. Not enough context
The text that you do keep in your web design should be informational and help to support your user in their decision making. For e-commerce in particular, it's important to make sure customers understand what you're trying to sell them. Make it compelling and easy to understand for someone who has never heard of your product before. Be straightforward, but don't be afraid to show some personality too.
3. Overly complicated forms
Forms are unavoidable if you plan on doing any kind of lead generation or communication via your website, but there's no reason to bog them down with useless fields. Asking users to give unnecessary information can be the difference between a new customer or an increased bounce rate. Do you really need someone's middle name or favorite color to send them an e-newsletter? Probably not.
4. Forcing users to register to access basic info
Don't let desire for information capture put up a wall between you and your users. Forcing users to register on your site to access information about your products and services is unnecessary and off putting. They will likely just leave and head to a different website. Think about it as give and take: only ask for their information if you are giving something in return like a newsletter or an order fulfillment. UX is a two way street.
5. Not leaving enough time for user testing
This may seem like a no-brainer, but the best way to make sure your UX is on point is user testing! It should be a priority and you should give yourself ample time for a few rounds of UAT in order to achieve the best possible results. You can always revise as you go and the more work you put in before the initial launch, the less work you will have to do in the long run.