For me it is the elegant design of the pistol which is prefectly sized for the .45 ACP round, like the Browning Hi Power is designed around the 9MM. It is slim, with round contours, not squared off blocks like the newer polymer guns. The grip fits me comfortably as does the grip angle. The pistol is the right balance of weight to absorb recoil, yet with the proper belt and holster is easily carried.
Once you get a few rounds downrange with it, the 1911 is easy to shoot accurately, and if you work up with softball loads, can be extremely accurate if you do your job.
It is also an aesthetically pleasing pistol that is easy to field strip without tools, and with a little education easy to work on yourself. Mine have been ultra reliable, accurate, and easy to maintain with the occassional recoil spring change and cleaning.