Well, if the pitch for the operation had been limited to "go save the Eastern Empire in exchange for cash and land grants" instead of ballooning into the Crusades, some smaller number of the younger sons of the aristocracy, poorer knights, and other military adventurers who made up a significant part of the historical crusades might have elected to become mercenaries in Byzantine service. The arrival of troops equipped and trained to the standard of Western knights, in this case under his command instead of pursuing their own ends, might have allowed Alexius to retake the Anatolian themes and push the Turks back.
There certainly wouldn't be enough men to achieve a monumentally successful operation like the First Crusade, which had carved out one of the wealthiest and most important centers of the Islamic world in one swoop, but they could have stabilized the situation for the Byzantines and, settled on central Anatolian lands recaptured from the Turks, formed the basis for a permanent "Frankish" military establishment under Byzantine control in the heartland of the Empire. Also, with no First Crusade, there would be no disastrous Fourth Crusade, which had effectively ruined the Empire. So in a best case scenario the Empire would be healthier, perhaps to the point of regaining its pre-Mantzikert borders in Asia Minor, and stabilizing them in a more permanent sense with the injection of new manpower.
The more profound effect, however, would probably be on Western Europe, because of the defining nature of the Crusades on medieval Europe.
"I am gravely disappointed. Again you have made me unleash my dogs of war."
--The Lord Humungus