Mostly its small things, one major one that took me by surprise but is now long fixed was:
If you sell physical goods and you put it in a box or wrapper, which you do because shipping, you need to have a membership with one of many private recycling firms that are part of one of the overarching systems (green dot, resy etc.). You never send anything to this company, this company never does anything directly for you. They only ask you once a year to tell them how many tons of cardboard, plastic, and metal you used to package your goods. Then you pay them according to that amount. If you are below a certain amount you pay a minimum fee.*
New regulations and changes are also a major pain:
One of the newer things is that you can't use google analytics on your webshop. If you want to use it it can only be anonymized, you need to state some legal BS in your terms and conditions and you need to make a direct contract with google in order for it to be legal...
The button that finally seals the deal has to have a specific text on which the lawyers are still fighting which of the options actually is legal to use.
For each of those you could receive a legal notice from anyone that is remotely competing (read: any other business) with a heap of legal fees attached that could put you out of business.
* Fun Fact: There's already a recycling surcharge on all the boxes you buy. The customer also pays for the recycling of his waste.