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

"...What is an ALICE pack like to carry and use?..."

As far as I, and my back, are concerned, an ALICE is one of the most uncomfortable things I have ever had to carry. Think one size fits none, super durable, therefore super heavy empty. There are worse packs around I am sure, but I know that there are better ones. The price can be right on them sometimes, that is the best I can say...

What are Alice packs with the frame like to carry ?
Simon

I recently purchased a Zitelli bag from ebags.com. It's 20X10X6 or so and holds my BOB gear with ease. (specs on the web page are incorrect) It's MOLLE compatible, has plenty of pockets inside and out and has both chest and waist straps. It was $30. If you go to their site and search on "MOLLE" it should pop-up.

Andy

PS, here's the link:
http://www.ebags.com/zitteli/z1_molle_pack/product_detail/index.cfm?modelid=87867

Horrible. I bought one at a garage sale just because I thought it looked interesting. I can't figure out why anyone would willingly carry a 30 year old backpack when there are hundreds or even thousands of backpacks on the market today which are much lighter, more comfortable, and better designed. I can't even say it's because it's cheap, because for what I see them selling for, you can get a decent backpack for about the same price. I think some people equate weight with durability, but for the weight of one of those packs, you could carry another spare pack inside your modern day pack and still come out ahead.

the arcteryx AC2 series backpacks are completely waterproof. they are REALLY nice but rather expensive

www.arcteryx.com

Are you talking about waterproof or weatherproof? The packs made out of Cordura 1000 are, for the most part, weatherproof. That's things like the Maxpedition packs, or Spec Ops. As I have mentioned elsewhere in these forums, I like to take a Star Trek approach...triple redundancy. Put the stuff that needs to keep dry in dry sacks, ziplocks, Lexan bottle...whatever works for you. If you're expecting a lot of water, you may want to get something with a drain hole in the bottom. FWIW, I just got a Max Pygmy Falcon II and started using it as a daypack. I really like it so far, although if you don't like the tactical look...
I got a khaki one, because I figured that was more neutral and less likely to draw attention (although one spry old woman wanted to know where I got it...thought it would be good for geocaching!). I wanted to get one in navy...even less conspicuous, but I got an awesome deal on the khaki one, so I had to get it.

I used mine quite a bit. But only for long day trips. I seem to be one of the few people that like it.I found it to be comfortable. Capable of carrying pretty heavy loads. Works well with a pack shelf to carry jerry cans of water. Check a local Army Navy store and ask if you can try one out in the store. I only ever used the med size. I guess it's a subjective thing as to if you like it or not.

Wonderful. They must have had my back in mind when they designed it. It carries heavy loads easily, has many lash points, and is one of the most comfortable packs I've ever had on my back, including many internal frame packs. As always, YMMV, but for me, ALICE is by far the most comfortable!

Having humped an ALICE around for the first few years of my service, I personally find it comfortable. Some aftermarket shoulder straps and belt (Tactical Tailor or Eagle) go a long way in improving the comfort of the pack. However the price of the new straps and belt exceed the price of the pack and frame. I really like the ALICE, I still have a medium ALICE that is currently in for some improvements/modifications at Tactical Tailor. With the price of the straps, belt, frame and modifications the price of the ALICE has skyrocketed, and I know from my research there are substantially better packs out there. I stick with the ALICE because it works for me, it's simple, durable and we have a long history together.

I will say, that I'm in the minority in the Marines who liked the ALICE, honestly, most hated it. When you utilize the frame the amount of internal cargo space you can use decreases substantially, perhaps by as much as a 1/3 of the volume without the frame. The frame itself, the middle vertical support is prrone to breaking at the top crossmember, especially when you throw it off of 7 tons or from frequent drops when there are halts on the hump. This is easily fixed with a bolt and nut though. I would have to say, if I never used an ALICE before I wouldn't spend your money on one, like others said, there are better packs out there for the $$$.