Well, it's of course a matter of taste, but I can't help thinking that boulevards represent the quintessence of what I consider as urban. It's on the widest and most swarming boulevards that you feel that special buzz of big cities.
Actually, the lack of such boulevards is particularly the reason why London is such a frustrating city to me. It pretends to be the center of Europe, if not the world, but it lacks that specific buzz coming from wide boulevards. Also it is true that the average building in London is very small (about 3 stories) which contributes to make feel London, at street level, as a relatively "calm" city considering its size. Actually, my sister and her husband like to say that what they love in London is that it has the quality of life of a provincial city with all the advantages of a large city. With time, I tend more and more to realize that, actually, this description isn't that wrong.
Anyway, I can't let anyone saying that all boulevards in Paris are similar. The athmosphere from a boulevard to another is widely different, even in the city proper. We cannot compare the feeling of the Boulevard Saint-Germain to the one of Boulevard des Italiens. Same goes with the Champs-Elysées that can't be compared to the nearby Avenue Foch. Boulevard Voltaire has nothing in common with the boulevard Montparnasse, and both are themselves very different from Boulevard Sebastopol. All these boulevards are the best representatives of the neighbourhoods they cut through ; and those neighbourhoods vary massively, from intellectual areas to more popular ones, from business areas to ethnic districts, and so on and so forth.
Anyway, I guess it's the same everywhere. When we discover a city as a visitor, we get firstly absorbed by the overall athmosphere of it. One needs to stay longer in a city to realize how different is each neighbourhood to one another.