First point-running form/body type. Look at all the greatest middle distance runners (The sub 1:43 guys ,sub 3:30 guys, sub 7:30 guys and sub 13:00 guys) and then compare their body types and strides to Alan Webb.
WEbb:
http://i.usatoday.net/sports/_photos/2007/07/22/webbx-large.jpg
Most top middle distance runners have long, flowing strides. They are relaxed even at high speed or when tired. They have very little upper body muscle-just enough to offset the motion of the legs and control pelvic motion. Any more is not needed, and is just extra body mass to drag around. Alan Webb thinks that it is needed, but it is not. When you are trying to kick off of 60s pace, it could be helpful. But when trying to kick off of 57-58s or run sub 3:30, it is not.
2nd point-relaxation. Webb has a short, quick stride that is often choppy. He doesn't relax most of the time, and never when it matters. He has a massive aerobic capacity but wastes energy because he is often working against himself. When Webb is on, he is very, very good. Perhaps good enough to equal the performance of an El G or Coe. Physically, he is there. But mentally, I think he is a mess. When he doesn't relax, things aren't going to plan or he doesn't feel good he seems to panic and then his form goes to shit.
Remember his 2 mile race when he ran 8:2x? First he came through the mile in 4:03-4:04 expecting it to feel easy. But he wasn't relaxed, so it didn't feel easy, so he panicked, and lost form even more. And ended up running like shit. And subconciously he must know why he runs like shit, but he doesn't want to admit that he panicked, so he says that it is inexplicable.
A hungry, well aligned and in shape Alan Webb is unbeatable at 1500m. But he doesn't seem to have the mental strength to run well when things are not going 100%, and that's what a true champion needs.