Shopify's missing a lot more than that for retail. I wouldn't use their current POS app for running more than a flea market table or street vendor. It's just too basic even for the smallest store.
You can't handle hundreds of SKUs by paging through product thumbnails at checkout; you need barcode scanning, and from experience, iPads and bluetooth scanners are a real pain to work with. Shopify also has no real inventory management (figuring out what to reorder, entering in the new orders as they get delivered, tracking cost of goods sold and spoilage, printing price tags and barcodes, etc). The couple of reports they've built are inadequate for proper accounting.
I hope they get there eventually, but the number and complexity of features they need for a small shop to run their POS on Shopify means they're at least a few years out.