Agree with almost everything i've read in this thread so far, the story is very undercooked with certain characters and issues playing a part in like 1 or 2 missions only, and without being wrapped up properly in the end. So many characters who were undeveloped - Stretch, Tanisha, Ron, Wade, Madrazo, The chinese guys etc. all played an important roll in 1 or 2 missions and then were completely forgotten about until late on in the game. Best example is Stretch: 1. First mission where he's fresh home, and its clear there is a problem between him and F but no real explanation why 2. gets them set up or something when they sell the brick on Grove Street (again poorly explained) 3. Don't see him for the remainder of the game 4. Last mission where one ending option involves a rushed assassination on him for no reason atall (Michael hasn't even met him and hes the one who suggests he needs to die) - it's actually quite mind-boggling how poor this was done.
Other examples include TP Industries, Ron and Wade's side story line where Trevor basically forgets about it all after about half way through the story (although he mentions it a few times later on in the game, it seems this was forced in to remind us it exists). Like someone above says, Trevor should have had management missions involving cooking, shipping and selling meth!