I think Harvey has rightly pointed out why the M14 was replaced--it was the Vietnam War more than anything else, not a failing of the rifle.  No, the wooden stock was not ideal, but the DOD had a good fiberglass stock ready to go and produced in large numbers (I have one on my M1A).  And another failing was also not the M14's fault--using one rifle to replace a battle rifle, a smaller support troop's rifle, an SMG , etc., all in one is simply a bad idea.  Blame that on the mentality of the DOD at the time.  They couldn't even do all that with the M16/M4, and had to have modern replacements for the M60 of our generation. 

But back to the Vietnam War.  If we had found ourselves fighting in Eastern Europe across wide plains, in coniferous forests, in hilly country and mountains, I think it's more likely the M14 would have been appreciated more, and very possibly kept.  Harvey said something along the same lines.  The rifle is a good rifle.  If one takes any good piece of equipment and tries to use it improperly, one can expect failure--but we shouldn't blame the rifle.

I'm the first one to admit that I have personal prejudices.  I don't like the M16/AR15.  I especially don't like the 5.56/223.  But I'm not just blindly criticizing a rifle or cartridge in order to pump up a rifle I do like better.  And I think a lot of M16/5.56 supporters criticize the M14 with somewhat weak reasons in order to build up their preferred rifle and/or cartridge.  I'm not talking about anyone here.  I think most everyone here debates these topics based on rational reasoning, whether we all agree or not.  But in the greater sphere of firearms fans outside this forum, you get a lot of "mine's better because I like it better" crap.  That's fine if we're kidding around, but not if we're being serious.