Normally, one small feature of a new version of Android wouldn’t deserve a post of its own, but this has been a huge issue that many people have had with AOSP for a long time. In fact, while there are other benefits to custom ROMs, the lack of a T9 dialler in AOSP builds is the single factor, for me, that necessitates the use of CyanogenMod (or equivalent).
The only Android phones that don’t ship with a T9 predictive dialler are Google’s Nexus devices. As custom and OEM ROMs have had this ability since the dawn of time, I honestly don’t know why it’s taken Google so long to implement the feature, but I was probably a little bit too excited when I found this out. Like zooming in Gmail was for a long time, T9 prediction is disabled by default in Android 4.3, so will require you to enable it in settings, but it will be so worthwhile.