To start with the best feature, it is LOCATION. It in the most literal sense couldn't be more convenient to the stuff Edinburgh visitors want to do. The front door is literally within 200 yards from about 9 or 10 great places to get a drink or meal. The apartment building is at the very base of the mountain that hosts the castle, and Royal Mile and Greyfriars are both within 5 minute walk.
Given the location, the second best quality is the VALUE for money. It's a nice comfy flat, and we found it to be great value for money.
Beyond that, the flat itself was SPOTLESSLY CLEAN and it has fast wifi, cable tv, and the typical amenities you need.
Should we ever find ourselves back in Edinburgh, I'd absolutely look to see if this flat was available before considering anything else.