Despite its ubiquitous presence at espresso bars, the latte is more American than it is Italian. The story goes that it was invented in California, to suit the American taste buds. Basically, your typical Italian cappuccino was too foamy -- and thus tasted too much like coffee -- even in pre 1950's America.
Which shouldn't come as a surprise, really. Americans have always drank more milk than anyone (as an aside, I should say that I'm totally not looking that up, and just assuming). Lactose intolerance is also much more common in Italy than the United States, thus Italians could only handle so much dairy with their espresso. Americans, on the other hand, could drown their coffee in it.
And they proceeded to do just that, and still do. Many now mask the taste of espresso behind even more veils, using flavored syrups and chocolate to craft vanilla lattes, mochas, and other disgraceful -- ahem, I mean decadent, *wink* -- drinks.
But even the most snobbish among the coffee elite must admit that, properly prepared, a latte is a creamy and delicious treat. Steamed milk provides a fluffy, creamy, marshmallow like texture, and accentuates the naturally sweet espresso with its own sugar -- lactose.
For those of us -- *raises hand* -- who would like to avoid digestive issues and throat swelling, there are always alternatives. The best I've found, thus far, is UNSWEETENED soy milk. The texture approximates dairy well enough, and the nutty flavor compliments the espresso quite nicely.
Regardless of the "milk" used, a quality latte must consist of two things: Good espresso and properly steamed milk.
Good espresso is essential, of course, because no amount of milk can hide a bitter shot. For lattes, it also helps to use a slightly sweeter espresso, thus allowing it to cut through the milk.
But a latte, as the name would imply, is mostly about the milk. Too often, a barista steams the milk in such a way that the latte is 95% plain ol' hot milk, and 5% arid foam. This is too bad, because a good barista (like yours truly) can steam milk to a velvety sheen. The texture should be consistent, soft, and creamy. There should be no large, visible bubbles. Instead, the milk should glisten like shined metal.
There are other factors to consider too, such as the aforementioned syrups (evil) and what fat percentage works best (whichever you like, but half & half is the correct answer), but those would make this post far too long.