Fenrir wrote:The most interesting facet of Hinduism, in my opinion, is its ability to adapt and change quite peacefully with each invasion into India proper, remain the predominant faith, and absorb invading peoples into the caste system. Nearly every culture that invaded India in the past eventually settled there to some extent.
They settled because of the ganja, perhaps? Why else deal with monsoons every year?

As to Hinduism's flexibility with other religions -- I agree, it is interesting how sponge-like it is. Another interesting point is that Hinduism and its offshoots have never organized armies for the sole purpose of converting others to its beliefs. In general, I think the most appealing aspect of Hinduism is that it is not primarily concerned with judgments and condemnation.
In a love song by Daler Mehndi, he sums up the attitude perfectly: "The world is a colorful place / it's not good nor bad." It is very tolerant of variation and diversity. The roots of this outlook on the world can be seen in the early Hindu texts, in which the "objective" of sorts could be generally summarized as coming to peace with the world as it exists, as opposed to waging war on it through moral judgments. This, I think, helps to explain the sponge-like nature of Hinduism, and also how it has persisted so well throughout the centuries (despite not spreading much).
With this in mind it will only take a moment to realize why militant and self-enforcing religions are the most widespread, and how this came to be so. Let us segue toward the teachings of an ancient Hebrew cult leader. Let us compare the "colorful world" interpretation of people having different ideas, beliefs, and practices to these statements from the Anointed One:
"He who is not with Me is against Me; and he who does not gather with Me, scatters."(Luke 11:23)This is a quick way to make enemies with everybody. You may recall former President Bush making similar claims, "You're either with us, or you're with the terrorists."
"Enter through the narrow gate; for the gate is wide and the way is broad that leads to destruction, and there are many who enter through it. For the gate is small and the way is narrow that leads to life, and there are few who find it." (Matthew 7:14)This sums up very neatly the sentiment expressed in the Old Testament, which is basically that the Israelites are the only righteous people and the rest of the world is wicked. The main difference between Judaism and Christianity, in this regard at least, is that Christianity seeks to change the world to become like itself, while Judaism is content to keep the "chosen ones" few in number.
This whole notion of "the way is narrow" is, in my opinion, entirely absurd. If you seek a particular, specific goal -- if you are training for something, for example, or hunting a certain prey, or something very specific like that -- then yes, you may have to adhere to a specific regimen. But if you look at the world, it is obvious that there is a great deal of variation in successful lifestyles within the human species (not to mention the countless other species which inhabit the earth).
But of course, Christ did not mean "life" as we know it -- which is to say, "life" which actually exists. He meant something that is actually the antithesis of life, which naturally "exists" only beyond death. Of course! Eh, all right ... rant over.