Actually, Laura, your question--why is it different--would have been fine if you hadn't added in the value judgment.
It IS different, for many interesting reasons, and it on e of my favorite Sondheim shows.
The biggest reason is that Sondheim was young and enthusiastically influenced by the comic voices of Larry Gelbart and Burt Shevelove. Gelbart had been trained in the live TV comedy world of Sid Caesar. Shevelove was a Yale-educated classicist who directed a lot of comedy on television.
They set out to translate the conventions of ancient Roman comedy into contemporary vaudeville and make a crowd-pleasing comedy.
So the "experimentation, innovation, perception, exploration and analysis" that Scripps mentions are there, but they're there in the overall concept, which was so much a combination of Shevelove's classicism and Gelbart's comedy, there wasn't much room for Sondheim to do anything except throw a couple of triple rhymes in every now and then.
But the entire script and score are written with such exuberance that I think you'll like it if you give it another listen or two.
It is VERY different from the darker colors that Sondheim attempted to paint with in his subsequent shows, but you can enjoy it for what it is.