In addition to the exclusives issue, the client and store were not particularly well made and were missing pretty standard features stuff like no shopping cart so if you wanted to buy multiple games at the same time you had to do them as separate purchases and stuff like wishlisting.
I remember Valve got quite a bit of dislike when they first introduced Steam as a requirement to play Counter Strike Source and HL2 (of course back then it was only for Valves own games).