Well, for one thing the dark ages probably wouldn't have happened, so scientific discovery would've had about 1000 years extra to develop, so by now we'd have technology that would be unimaginable for us today. But on the other hand, we could easily have been stuck as a bunch of small tribes without religion.
I think it just depends on what was used to fill the void that religion currently fills. This is an interesting concept to ponder, since religion has obviously caused many attrocities, but the good that it does is often diffused. Without religion it would likely have taken civilization longer to organize itself, and organization on some level is neccessary for economic efficiency. Religion also makes for a convenient way to explain morals without requiring everyone to have a degree in sociology. To people with little education, it's difficult to connect having personal morals to positive advancement of society since humans tend to be myopic and selfish, especially in poor economic conditions. Instead of trying to explain the benefit to society that an individual can have by living honestly, it's easier to just say "God sees everything and will burn you in hell if you step out of line."
As an atheist I'm reluctant to admit it, but it's entirely possible that the concept of god has a place in a developing society. Without the immediate trust and comonality that religion can provide two strangers, it would likely have been very difficult to form societies larger than a small tribe where everyone knows everyone else. It may just be that a society has to progress from small tribal groups, to large religious groups, before it can become a global atheist (but moral) society. Sometimes it's tempting to skip the painful middle steps, but it may have been necessary in order to become organized on a sufficient scale.
