Rol_Lei Nut
Member
THings to ensure your pack choice has - an *included/attached* water-proof cover (this is a requirement not an option), plenty of space - 30Ltr main-bag volume should be more than enough (though a 40Ltr pack isn't out of the question depending on anticipated weather), stiff/thick shoulder padding (not the soft, squishy garbage that will ultimately hurt/chafe your shoulders), a reasonable waist-belt, external mesh pockets, loop/daisy-chain attachment points, robust zippers (YKK 10 is perfect size for the main-volume zipper) and an internal sleeve/external access for a water reservoir (Camelbak/Platypus, etc). These are the basic "features" i look for in a day pack.
Or better still, a main compartment with a cover instead of a zipper.
Some very good 30+ litre bags don't have built-in rain covers: a separate rain cover can be bought or even (what I actually prefer) an internal waterproof sack.
Most of the specifications Fmajor listed should be met by any reasonably modern bag from the companies I listed in a previous post.