No SEF URLs and meta data capabilities out of the box and for paying customers as well. Your competitors will dominate you in the search engines.
Using Google Adwords? Well, you can kiss your quality score goodbye, because content does not showup on page the way that Google Adwords needs to see it... even with inline seo enabled with silver membership.
Looking to retarget / remarket to customers? You better forget that as well. There is no way to add codes to product specific pages. Again, your competitors will dominate you in the competitive search game.
If you're looking for an easy to install alternative shopping cart, Ecwid is for you. But if you are a serious business owner looking to make money, than it's best to look else where for a more robust shopping cart platform.