The biggest difference between Hinduism and Buddhism

[Okay, maybe Mike Johnson is a competent parliamentarian.]

Correct for Theravada but that's not necessarily right for Mahayana and Vajrayana, whose understandings of identity veer in many cases somewhat closer to that of Hinduism. As is so frequently the case I'll wait for a while to see if anvi wants to get into more detail about this and if he doesn't I can explain further.

I'd say that based on my understanding of the religions the main difference that's theoretically true of all or most Buddhisms is the decoupling of the concept of dharma from notions of caste or social positioning. It's important not to downplay how important the interrelation of these concepts is to most forms of Hinduism. They also differ notably in their understanding of deities--not necessarily their existence (this is a common misunderstanding of Buddhism that, again, is only arguably true of certain strands of Theravada. Most Mahayana and Vajrayana sects are very much polytheistic in the crude sense), but where they stand in the cosmic league tables, as it were.

[Okay, maybe Mike Johnson is a competent parliamentarian.]

They also differ notably in their understanding of deities--not necessarily their existence (this is a common misunderstanding of Buddhism that, again, is only arguably true of certain strands of Theravada. Most Mahayana and Vajrayana sects are very much polytheistic in the crude sense), but where they stand in the cosmic league tables, as it were.

Dont understand the last sentence.

In most forms of Mahayana gods are admitted to exist, but they aren't thought of as anywhere near as powerful or important to the functioning of the cosmos as in Hinduism, and actually being a god is considered less desirable than being human from the point of view of Buddhist practice, since the existence of a god is defined by a false sense of contentedness.

[Okay, maybe Mike Johnson is a competent parliamentarian.]

Also, most Hindus and Buddhists don't literally believe in reincarnation, and see it more as a metaphor for how we are really all the same after you strip away all the superficial characteristics of body and personality.

A lot of people in Southeast Asia are going to be very surprised to see you say that. 

Well, there's always going to be a disparity between the views of the average person vs. the theologian.

As somebody who aspires to be a theologian, I submit that the views of the average person are of significantly greater relevance in discussions like this, and they're by and large what I've been discussing in this thread so far.

[Okay, maybe Mike Johnson is a competent parliamentarian.]

Why? Few laypeople in any religion know why they believe what they believe.

I'll assume you wouldn't like an appeal to populism as an answer to this question so I'll just say that it's because the actions of laypeople generally have more observable results in terms of relations between different religions than do the actions of theologians. A theologian's importance is, with the obvious exceptions like the D.T. Suzukis and Thomas Mertons of the world, more internal to the religion, which means that it's entirely suitable for some types of discussion, just not, I think, this sort of cross-comparison (at least not on a forum ostensibly about politics!). Emphasis on generally, since there are certainly things that theologians, or ordained leaders who may or may not have specifically theological training, can do to influence ecumenism or interfaith relations on their own initiative when they really want to.