I've been thinking about this quite a bit. Here are my thoughts.
1. The design is somewhat complicated to manufacture, with some parts that are fairly delicate.
Specifically, the whole barrel-linkage thing. Yes, I'm using a Glock for comparison, but only because the Glock uses Browning's refinement of the same design. The 1911 barrel has a swinging link. I don't know how often it breaks, but it seems awful small compared to how high the forces are slamming around in the gun. It's also complicated to machine and assemble (compared to the Glock barrel).
The locking lugs on the barrel are completely unnecessary. Why not just have the chamber area lock into the ejection cutout like on the Glock or any of a dozen other modern designs? The modern design won't suffer from rounded locking lugs in the event of a weakening mainspring or overpowered ammo. Not to mention it's cheaper to manufacture for both the barrel and the slide. The modern design can't clog up with carbon or foreign material and prevent lockup like the barrel/slide lugs can.
The plunger tube is well-known to break and disable the pistol. It appears completely unnecessary to the design of the pistol, and I've always wondered why the heck it's on the gun.
2. Disassembly is significantly more complicated than modern designs. I'm sure it was a great innovation in simplicity at the time. It's not particularly difficult. But it's just not as easy. Even adding a disassembly notch on the slide (similar to the Hi-Power) would greatly help things.
3. Maintenance is required more frequently than modern designs. You can't just put on a little slide grease and a dab of oil here and there and be okay for months. Heck, I haven't lubed my Glock 19 (daily carry gun) for a long time.. I fired over 2500 rounds with no cleaning or lube over the course of 6 months, and still no malfunctions (I carried another gun while doing this test).
I believe this may be a result of the longer slide rails on the frame, as well as the larger surface area of interacting parts as compared to the Glock. Less surface area means less friction. It doesn't take a whole lot of rail to keep the slide on.
4. Extractor. It requires tuning. No part on the gun should require tuning to work correctly. A monkey should be able to slap a replacement part in the gun and have it work correctly. For various reasons, an external extractor doesn't work well in a 1911 - I say redesign the slide to use a more robust extractor.
5. Reliability. Yes, I know, you 1911 fans are going to say that your 1911 works perfectly. But there's a lot of them out there that DON'T work perfectly. The 1911 lacks what is called "robustness" in Mechanical Engineering. Robustness is the measure of a machine or part's ability to operate successfully in a wide variety of input conditions. For instance, a gun which can cycle fire without malfunction any .45 load from 185gr @ 600 fps all the way up to 250gr @ 1000 fps would be a very robust gun - whereas a gun that will only function with 230gr bullets traveling between 830-840 fps is not robust.
What I'm saying is that there are a number of variables which may conspire to cause a 1911 to malfunction - an extractor, feed ramp, magazine follower, magazine spring, slide stop, grip safety; shape/size/power/specs of the ammo; lube condition, etc. Any of those slightly out of spec can cause a 1911 to malfunction. Some of the better modern designs can be quite robust, working even with completely broken or missing parts, bad ammo, no lube, etc.
If I were to redesign a modern version of the 1911, it would have a linkless ejection-port single lug barrel, shorter slide rails, simpler disassembly, a from-scratch external extractor, etc.
