Not wanting to be irreverent, but perhaps for some people who don't find anything of to him genuine worth in the teachings of Gautama, he could also be styled Gautama the Terrible, for inspiring so many men and women to give up their best years and all their life years in pursuit of emptiness in isolation from home, family, and community, that is, in the sangha (read that monastery).
Well, Susma, I don't know if such sentiments are realistic, at all. Firstly, let's compare 'Gautama the Terrible' with somebody who is, let's just say, a bit more qualified for the superlative. How about 'Ivan the Terrible', who you, yourself, mentioned?
Inspired by 'Buddha the Terrible', many people became monks or nuns. Why? To seek spiritual liberation. At the hands of 'Ivan the Terrible', thousands upon thousands were executed, including his own son, the prince, who was felled by a blow to the head with Ivan's staff. Why? One day the prince's pregnant wife wore clothes that Ivan apparently did not approve of, so he beat her so severely that she had a miscarriage. When Ivan's son came to him infuriated over the incident, Ivan stuck him dead. Let me assure you that 'Ivan the Terrible' may very well have been 'Ivan the Great' to some, but not to those he had slaughtered. Not to mention that in this modern era, only a few hundred years after his death, Ivan can unquestionably be called terrible...his greatness, it seems, is largely a product of nationalism in times when society didn't really have a huge problem with constant, successive slaughters. Things have changed nowadays.
Now granted, there are more types of violence than just physical. And frankly, I guess one COULD 'style' the Buddha to be 'Terrible', but no more than anybody else who had a profound effect on people's lives...so, that's Christ the Terrible, Muhammed the Terrible, Socrates the Terrible, Confucius the Terrible, and so on.
Suffice to say, there is as much realism in the idea of 'Gautama the Terrible' as there is in thinking of delicate Venus Fly-traps as beastly, man-eating, trees. Taking the whole of human history into account, there have been innumerable people who rightfully earned the title of 'The Terrible', and they've been granted that title during their lifetime by people who could could attest, first-hand, to their genuine wickedness. The Buddha does not number among these.
Monasticism has had a place in pretty much every civilization that has existed; Buddhism can hold no claim to being unique in that respect. And furthermore, it would be far too hasty for someone to assume that his or her values as far as the 'proper utilization of our best years' are necessarily universal. Perhaps their best years were well-spent outside of a monastery, but would it not be reasonable to allow for the possibility that others may have benefitted very much from their time as monks or nuns? Is it realistic to believe that the whole of monastic history and lineage is entirely one of misery and wasted life? I don't think that can be said, so clearly there must be something to it, no?
By the way, I realize that it may seem as though I am evading your question as to "Why the Buddha is so great", but not needlessly so. The fact of the matter is, you seem to be starting off tilted pretty drastically to a preconception that his greatness is a fraud, yet you seem to be asking about his teachings as if you don't know enough about them to be able to say either way. I am curious of why this is, but that is beside the point, really. It is, of course, beneficial to question great things sometimes, but the Buddha's greatness is not his doctrine, and if you're caught up on trying to either affirm or deny him as being 'great', you'll miss out on what he really has to offer.