Glock provided the people with what they wanted; the ability to change the back strap. What were behind the other changes? Now, compatibility is sacrificed.
-Texture (Although there is the RTF2, but I think with all the stipple jobs out there, people were asking for this)
-Enlarged Reversable Mag Release, if you keep it right handed you can use older mags. (I believed people asked for this indirectly judging by the aftermarket parts out there)
-Trigger Bar has a 'bump' on it now, although some gen 3's are shipping with them now. The 'bump' looks like it is there to allow the trigger bar to use more of the firing pin block by having the bar travel in a more straight line. This also supposedly adds a bit of roughness to the trigger. It is backwards compatible to a non-bump bar. (seems like Glock is updating this on all guns going forward)
-Dual Recoil Spring as mentioned (supposed to reduce recoil, no one asked for this LOL), there is a retrofit part that is aftermarket so you can use gen 3 recoil springs.
-Trigger housing is different which also causes the connector to sit at a slightly different angle, it is theorized that this causes the trigger break to require a bit more effort. (by-product of grip changes?)
I have a Glock 17 Gen 4, no failures yet..... I have only really used Federal 115gr, 124gr and CCI 115gr Brass and 115gr Aluminum. Mine has the '02' updated recoil spring assembly.