all of the above.
i check them out if they are easy to navigate- prices are not ridiculous- have a nice variety/selection- offer flexable shipping options-
if they load too much 'stuff' on a page so it takes forever to load- i move on-
the best ones are ones that allow you to choose how much to see (as in 10 items showing- click here to see 20, 50, all ect.)
if the shop seems to waste my time i will not go back
so i guess time/ ease of navigation- user friendly is top on the list- with presentation being next- then prices- then shipping/ shopping cart features.