Yes and no. Of course there are distinct theological differences. Episcopalians in general have a very different sense of what happens at the Lord's Supper than do, say, Presbyterians. But in practice, the denominations overlap quite a bit. A quick look at the "family tree" of any modern denomination will reveal common ancestors and forebears among all the denominations. And this is born out in the life of Protestants today, too, with the vast majority showing a great deal of loyalty to mainline Protestantism generally, but not a great deal to the particular denomination with which they happen to enjoy affiliation. (Most Americans are more loyal to their brand of toothpaste than to their denomination!)
Historically and culturally, in fact, most of the mainline denominations in this country represent primarily ethnic differences: as people from Scandinavia or Scotland or Germany immigrated to the United States, they formed separate communities which separately interpreted the teachings of the great reformers and eventually established different churches in North America. As those ethnic and cultural divisions have lessened over the decades, though, so have the distinctions between denominations. In fact, today, many of the mainline churches have agreements or concordats of relationship and communion with one another in an expression of this common identity.