Well: aliexpress is the end customer portal, while alibaba is the B2B portal for bigger purchases (1000pcs and up).
I'd like to compare it to ebay in chinese with just the option to buy it now!
I guess one needs to be very patient when buying from asia. Usually there is nothing wrong, sellers are friendly folks as long as nobody do upset them with wrong wording.
I don't know how aliexpress buyer protection works, but I guess they are using an escrow system.
Can't give them a bad name. All orders from different sellers went smooth, but took their time (talking in weeks).
Usually on Ali you find the best price. Alternatively have a look at banggood with their incredible customer service and world wide free shipping.