Let's see:
1. Much less ergonomic. That's why many of the M14s that are being refielded are seeing completely new stocks.
2. Less accurate. YES. The piston design of the M14 puts pressure on the barrel, which significantly effects shot groups. By comparison, the M16 has just a little gas tube that inflicts minimal deflection on the barrel during recoil. The M16 is overtaking the M14 at ranges out to 800m with the right ammo. Even them, combat engagements with rifles past 300m is rare and under 100m is the norm.
3. Less adapatable. Even with new stocks, the M14 is a pain in the ass to mount a decent scope, let along lights, lasers, etc. I can bolt on a $200 tube to the M4 but I have to completely re-stock the M114 at a cost of nearly $1000. If we wanted a 7.62mm rifle, we should have adopted the FN FAL.
4. Even if you go with a "bush" model M14, the 14.5" barrel of the M4 carbine is much easier to use in a MOUT envrionment. It's lighter weight means it's easier to maneuver quickly.
5. Controllable under automatic fire. No contest. The M14s ability to use this feature is minimal. Meanwhile, I can keep the M4 controlled on a 25M target through the entire magazine.
The M14 was one of the shortest-lived issue weapons in US military history, while the M16 platform remains the longest.
I'd put up an M14 against an early M16 any day. I remember the early M16 with the wimpy barrel was very inaccurate if fired using the sling as we were taught in USMC rifle training. The tension would bend the freaking barrel enough to make it impossible to find a zero. The Marines did manage to get the M16A2 barrel strengthened as well as give it decent sights. I think I noticed all the target M16 derivatives have floating barrels now as well.
I liked my HBAR very much, it was a very accurate rifle, regardless of the stopping power. My Mini-14 on the other hand was useless beyong 50yds, but was a cool little rifle.