Three differences come to mind.
1. Without Paul, Christianity would not have spread as fast or as far as it did in its very early years. Paul was really the great salesman of early Christianity. Without Paul and his travels and efforts, Christianity would probably have remained more localized to the area we call The Holy Lands for a longer period of time.
2. Without Paul's influence and interpretation of Christianity, and his extensive writings, it would be Jewish Christianity incorporating many Jewish traditions, views, etc that Christianity as we know it has moved away from. Without Paul, Christianity might a sect or form of Judaism instead of being a separate religion.
3. Without Paul's letters and other writings, The New Testament of The Bible would be much smaller than The New Testament as we know it.
I think it is very safe to say that without The Apostle Paul and his influence, Christianity throughout history and still today would be quite different than the Christianity we know in OTL.