Not an AR expert, but I'll take a stab at it from what I've learned so far.
I think regular uppers have no "feed" groves cut into them, just the barrel extension has grooves in it to feed ammo.
The M4 feedramps have deeper groves cut in the barrel extension that are extended into the upper receiver. If you don't have a matched barrel extension, you very well may have serious feeding issues, which defeats the extra cost of the M4 ramps in your upper.
FWIW, I've been told you can have 3 successful combinations of upper/barrel extension, and 1 failure:
Good: M4 upper/M4 extension
regular upper/regular extension
regular upper/M4 extension
Bad: M4 upper, regular extension
Check out this thread that shows an upper with M4 feed ramps for a visual:
http://www.ar15.com/forums/topic.html?b=3&f=4&t=381786
In the second post, at the bottom of the receiver, you can see two little notches which are the M4 feed ramps.
ar15.com has lots of good info, just browse through the AR section. I'm just starting my first build, and have learned a lot from that site.
Edit: See post #7 (and the rest of this thread) for good comparison/discussion -
http://www.pafoa.org/forum/rifles-42...pros-cons.html