I've worked retail for many years and a few months ago I had a woman enter and ask me at the register if we carried a particular style of jean that was extremely popular at the time. She was desperate to find it and she had checked several other stores, including branches of our company. She was frustrated with our other stores and only checked in with us again as a last resort. She attempted to explain the jean she was looking for, giving only the brand and a relative color. I was able to use our point of sale system to search for the jean, find it, and determine exactly which stores had it in stock, including all stores on the entire west coast for next day delivery. We had one in our store, I found it, and we rang her up without the need for her to stray any further than the counter. She bought two pairs, which means that by not having as efficient a POS system as us, the other stores lost out on $350.
Training
One of the most important things of having an efficient point of sale is having properly trained employees with the technical know-how help the customer the best way possible. Most clerks know only the basics, and any sort of hiccup or change from simple tasks will confuse, delay, and possibly terminate the sale right there.
Accurate Inventory
Every POS system needs to be modern and up-to-date. It may be an inconvenience for a guest to wait for a clerk to check inventory, but it will be a big problem if they are forced to go to another store and they don't have the item.
Hardware
POS systems need to be up-to-date, reliable, easy to use, and networked to your other stores, even if they may be slightly more expensive.