A heavier barrel will increase the heat capacity of the barrel by adding mass, ie, more heat energy is required to raise the temperature by one degree. Therefore, the new M-4 will fire more shots before the chamber heats up to extraction failures.
A piston gas system gives a sharper tap on the bullet case during extraction, compared to the current gas-impingement design. That higher transient force may be enough to overcome the case "sticking" to the heated chamber, further improving reliability.
So these changes are a good fix to the M-4. However, we still have the problem of excessive heating. The fixes only postpones the inevitable. I wish the Army would go to the root of the problem and either 1) slap a cooling fin/heat sink on the barrel nut, or 2) go to a longer barrel (16 - 20 inch) that the 5.56mm round was designed for.
Hat tip: the Firearm Blog.
PS: Added links and tags.