Hi Tom, a few suggestions from a guy who carried his laptop around with him every day:
A strap on the back so that you can strap the bag onto the extendable handle of a wheely bag. For those of us who travel a lot with their laptop, it's useful to have a compact bag which you can use every day to go to and from the office but can also use it when running through airports with your wheely bag. I guess this strap can double as the 'sweater strap' you mentioned earlier.
Room for a power supply. I know very few people who travel with only with their laptop. The power supply is usually always there and a pocket which caters for it without creating a huge bulge would be very nice. These days you can get slim laptops, slim PDAs,slim cellphones, slim iPods but the powersupply is always a brick and results in an unsightly bulge!
The top-loading zip is perfect for a laptop bag - quick but secure. If you are going to have a flap, it would be good if you sew it to the front so that we don't have to open it to get the laptop out.