You summed up the "problems" (for me) with Horizon quite neatly right there. Part of the overall charm of A Song For You, for example, was the sheer variety of sounds and material there. You never knew what you were going to hear next. Horizon in contrast is kind of boring. While it's very well recorded, as Harry said, the excitement and charm are much diminished. They almost sound TOO professional.(No instrumental fillers, no lead vocals by Richard, it's all just Karen's superb lead vocals).
It is ballad-heavy, even a bit draggy, as has been pointed out.
Longtime readers may remember that I was in the record-selling biz throughout the Carpenters heyday. As a music retailer, I thought it was a huge mistake to put "Please Mr. Postman" on the Horizon album. Since it had been a massive hit single, it had been played to death on the radio and even fans were growing tired of it by the time the album came out (not to mention casual listeners who had grown to hate it!) It would have been better if they'd put in another uptempo-type song, but not an oldie. They should have planned to save "Postman" for the next greatest hits album -- like Elton John did with some of his mid-'70s singles.