I ran across this question, and I thought I'd add my two cents since some of my work has been mentioned here.
I agree with most of what's been said, but it's important to remember that most everything mentioned represents heuristics. That means that while they're good principles to follow, there are likely times when (a) specific rules should be broken, and (b) there will be contradictions between rules. The trick is being able to weigh conflicting principles and apply them to the appropriate degrees (as you noted in a previous comment).
In the end, it's a matter of balancing the business requirements and user needs in a way that produces optimal results. And in the real world, I find that this can never be achieved through heuristics alone.
Here's an example: I recently finished POS designs for Subway, Wendy's, and Starbucks (see Case Studies at POSDesigns.com). All of these designs used solid heuristics, but all of them came out very, very different because of differences in the business goals and requirements, the users' needs and background, the environment in which they work, the technology being used, and a whole host of other differences.
You can never create a great design in a vacuum. For each of the clients mentioned above, I traveled around to a lot of different types of stores in multiple countries to get a feel for how the users' worked, how the systems would be used, how customers ordered, etc. All this information - along with sales and other data provided by the company - was invaluable in creating a highly usable solution.
Here's another example: Guideline #3 you provide previously ("Support Handedness / Dexterity") is fine as a heuristic (though I have to say that I question the conclusion of swapping simply OK/Cancel). But in visiting Subway stores, we discovered that in that context, the location of the register actually plays a greater role in the hand employees prefer.
In other words, registers that were squished against a wall on the right side tended to produce left-handed users, even when the users were right-handed for every other task. This had implications for the way we allowed the UI to be flip-flopped...and who had control of it. There are tons of examples like that, but we never could have achieved the gains that the user interfaces have produced - like 90% reduction in voids, near zero training, increased speed, accuracy, and check sizes, etc. - by following heuristics alone.
One more point (sorry...you've got me going now :-). Many times heuristics are incomplete without more data as to how to apply them. Consider your guideline #11, "Conversational Ordering". There's much more to this guideline than just providing flexibility in order entry. For instance, one of the many things you have to consider is that not all paths should be presented as equally probable.
We analyzed the way Starbucks' customers ordered in various locations across the United States and United Kingdom. Then, we optimized the system for the most commonly spoken patterns. If we had allowed all paths to have the same "volume", we would have sacrificed usability in other areas, since the design would have appeared more cluttered. The new POS system now supports almost all possible order patterns, but the most probable paths are presented at a higher "volume" than those that are less probable.
OK, it turned out to be more than two cents, but the bottom line is this: If you have a chance to visit the environments in which your POS will be used, analyze customer/employee interactions, etc. ...you should take it. Contextual observations and analysis are invaluable in correctly applying heuristics to your situation.
Good luck!
Dr. Kevin Scoresby
FYI - I'd enjoy talking further about this if you or anyone else in the group would like. My office phone number is on my "About Us" page at POSDesigns.com, or you can use the form to initiate an email conversation. Feel free to call anytime during business hours U.S., East Coast Time.
