Turnlock closure on rounded flap or full straight flap (halfway straight flaps often tend to look a little crooked), back slip pocket, inside zipper pocket and slip pocket under flap, buckle-mounted straps, not to big and not too small (though I adore my Dinky and wear her all the time). I don't mind other forms of strap attachment, but my least favorite is a permanently attached strap with square D-rings.
I have many that come very close to being perfect, but there's always something. If the Rambler's Legacy had a rounded flap or the Saddlery had a back slip pocket or the Manor had buckle-mounted straps or...

But then I'd probably get the bag with perfect specs and be done, and where's the fun in that?
Also, I just excluded all zip-top bags and that's just unfair. I love my Chester for example.