One reason the 734 never really took off with most -US based carriers- is the limited range and performance the airplane has, particularly with carriers that have hubs at hot and high locations like Denver, Phoenix, Las Vegas, etc. Carriers targeted (like UAL in 1992) wanted to perform nonstop transcon flights between East and West Coast with an airplane that size but could not with the 734. It was entirely dependent upon which niche a certain carrier operated. The -400 does excellent on short to moderate stage length routes, shuttle routes in particular, but longer routes it just doesnt have the capability without a severe payload restriction. A major reason why is it's wing combined with the extra weight it's hauling.
It's excellent in the regional role', finding homes with launch customer Piedmont/USAir, ProAir (on the verge of going belly up I've heard unfortunately) and Alaska (good sized fleet there). Launch customer Piedmont Airlines ordered quite a bunch of them in the late 80s and planned to operate them on high density short-medium range routes, which is exactly where it's purchaser USAirways operates them today. It didnt seem to take off with the other major US carriers though. You'd think they would have been a shoo-in for Southwest, but they seem content with the -300/-700 and -500 sized airplanes. The -400 would have required extra crew to work and they are sticklers about a very cut and dry operating plan. I have heard the performance issues with the -400 factored in a long time ago particularly for hot and high destinations like Phoenix & LasVegas and the payload penalties played a role. They do have the capability to convert to the -800 on this new order they just placed.
Further footnote regarding performance issues: Friends of mine with connections at BA maintenance claim they werent overly fond of the CFM engines on the -400 though on various issues and it played a role in their decision to opt for the A32X and IAE engine combo.
I've flown on China Airlines 737-400s and thought they were fine overall (it was a light passenger load each time I've flown them) configuration with the fold down monitors. They employed them on mostly short-medium range regional routes however.