IDK if anyone has noticed but the B737 isn't a baby anymore she is actually growing very fast...

http://www.airliners.net/open.file/741285/L/

First nicknamed "FLUF" (Fat Little Ugly Fella). the 737-100 did not sell very well and Boeing thought about deserting the whole project!...then with the fuselage stretch, the re-surrected 737-200 made history and paved the way for the most succesful airliner-series...every 5 seconds there's a 737 roaring down the runway taking off somewhere in the world 

If you ask me, the 737 stopped being a baby in 1988 when the 734 was introduced 
Technically inferior to the Airbus A32X, the 737 still holds a slight edge in sales, thanks to the many new LCC airlines which tends to choose Boeing over Airbus 

Yes they are, the hardware (computers) found in the A32X costs half a fortune, while the 737 still uses mechanical wires 

I was at Vegas this summer and watched jetliners landing at McCarran from a "visitor" drive in spotter parking area. (off the south road). Jet after jet landing were 737's with various liveries. Of course that could have been the time for 737 landings...(about 5-6pm Sat.)

The 737 has earned its place but when  it comes to just a great flying machine loved the world over by pilots who don't mind reading charts and tuning radios, who love feeling the yoke in their hands and the pedals under their feet, the DC9 has to take it all.  A very safe aircraft and actually since boeing started computerising the basic design it hasn't been the same since.
I must offer a counter point to the 737 being the low cost airline jet of choice.  I know one airline headquartered in NY that flies nothing but A320's an A340's due to the service economy and ease of pilot certification and ease of maintenance.  Jet Bue considers the airbus fleet one of the keys to it's success.  And that isn't easy for me to say being a Yank who bleeds red white and blue and grew up on John Wayne movies, lol.