I wasn't knocking Sondheim yesterday, but, rather, saying something about the different eras he, Hart and Rodgers & Hammerstein wrote in.
In the 20's and 30's, cleverness, especially in rhyme, was prized. Hart brilliantly turned out real emotion that was simultaneously clever "Only my book in bed knows how I look in bed." In those days, writing songs that could only come from the particular character singing them wasn't relatively important (although, as was pointed out, Hart did that, too).
In the 40's and 50's, starting with the revolution of Oklahoma!, audience expected love stories with songs unique to their characters, and ones that moved the plot along. While I like Rodgers & Hart songs better than Rodgers & Hammerstein songs, as SHOW-writers, no one tops Rodgers & Hammerstein.
Sondheim's Company was fairly revolutionary in 1970. As I said, he's got cleverness in spades (playing card analogy alert) but the era he wrote in didn't particularly prize cleverness as much as other virtues. I love "She sits at the Ritz with her splits of Mumm's and starts to pine for a stein with her village chums but with her mitts 'round a Schlitz down in Fitzroy's bar, she thinks of the Ritz - Oh, it's so schizo!" ... but that song's not as well-known as it would have been if it had been part of a hit like Babes In Arms.
In working on the Bock & Harnick revue last year, I put together something I called the "Elation of Love" medley: songs that express how wonderful it feels to be in love. Oscar Hammerstein's I'm In Love with a Wonderful Guy is the type of song his protege Sondheim would never consider writing. For Sondheim, illuminating the euphoria of romance isn't a strong suit (in this sense, he's weak in hearts). But, as someone writing in a post-Oklahoma era, he of course digs into the psychologies of his characters, and he does this extremely well (like diamonds).
There's an obvious difference between the revolutions of Oklahoma and, say, Follies, which is that the earlier audience immediately embraced the change. Oklahoma was a hit of unprecedented proportions; the public loved this new more-serious story where the songs fit the time, place and character. Around the time of Follies, a younger composer (with the same birthday), Andrew Lloyd Webber, burst on to the scene. Now, if you know me at all, you know I think Sondheim is ten times the songwriter Lloyd Webber is, but the audience doesn't agree. ALW wrote hit after hit. This leads me to wonder if the public was/is ready for musicals that don't contain traditional love songs. Is it really so surprising that a protagonist-less musical about the westernization of Japan didn't have a long run? Theatre-goers crave romance, which is why The Phantom of the Opera continues to run.
Finally, I'll repeat something that I don't think was understood. Yes, Sondheim often writes very emotional songs, and these move me greatly. Joy's rendition of So Many People in her cabaret act; Guy's What Can You Lose on the Sondheim movie song album; Losing My Mind in countless settings. It's the whole shows, in the theatre (where they were meant to be heard) that often pull me up short. It's why I see him as the opposite of his mentor: a great SONGwriter, a less good SHOWwriter. (He hasn't got a lot of clubs.)