As everyone else has said, LOCATION. It's amazing. I love taking buses in London & this flat is right in between two streets (Upper Street & Essex Road) with really useful bus routes. Easy walking distance (like 3 or 4 minutes) to great restaurants & grocery shopping.
Also, very comfortable bed. The washer/dryer combination actually does dry clothes, unlike so many combo machines. The flat is nicely decorated & well-laid out. It is up two flights of steps, but they're easy stairs with good lighting & handrails. Only a lot of luggage would be an issue. Well equipped as far as dishes, pots & pans, etc. Also basics like oil vinegar, tea, some spices. Good lighting & plenty of outlets (some via very convenient power strips). Good wifi.
The bathroom is a bit narrow and the tub is a bit high, but by the second morning we were used to it. Would absolutely stay here again.