Sacs are rapidly becoming zip closure and outside pockets are dissapearing too. If you want flap closure and pockets an alice pack might be your best bet.
Big advantage of the drawstring and flap is that you can turn a day sac into a weekend bag just by stuffing more under the flap. My work day bag has carried enough for working away for weeks this way. A zip closure can't be extended. And tying stuff to the outside doesn't work as well as you might think.
Even better if the sac has a storm flap. Tactical tailor will fit one to your alice pack for 19 dollars.
from their web site:
"The storm flap is a fifteen inch extension sewn to the inside top of your ruck. It has a draw cord closure which makes packing easier, and ensures that nothing falls out. For strength and durability, it is constructed out of 420 denier pack cloth"

If you use a bivvy bag (not sure what it's called in the US, it's a bag big enough to sleep in) you can tie off the bottom to make it the length of the sac and use it as a waterproof liner. Then you always have a bivvy bag with you, which you should have anyway.
Simon