On its own, the 743 isn't a bad aircraft. However, it doesn't always compare favorably with its predecessor (the 742) or its successor (the 744).
The 743's range is inferior to that of the 742, a problem for airlines operating the aircraft on very long-haul routes. The 743 is less well suited for carrying cargo because of the additional lower ceiling behind the forward hump (as compared to the 742). That's why dedicated 744 freighters have the 742's short hump.
The 744's advanced-technology engines offer much better fuel economy over those on the 742, which adds to a lot of fuel on a long flight. The 744's two-crew cockpit's advantages over the three-crew examples found on the classics are self-evident.
The aeroplane has unveiled for us the true face of the earth. -Antoine de Saint-Exupery