Very few games are fun in their first playtests. Even if you have a
clear idea as to what the game is about - and "Left 4 Dead" didn't
have that - it takes a lot of iteration to find the right balance with
the gameplay. Plus, early playtests are usually missing all the
graphics and sounds that give a game its unique character.
But if one thing has become evident over the past few years, it's that
Valve is probably the reason the game was as fun as it was. Turtle
Rock Studios may have come up with the idea, but it was Valve's
experience (and financing) that made the game shine. Lacking Valve's
support, the original "Left 4 Dead" probably would have been awful...