Word wrap is turned off by default in Kile. Since it is typically used for typing long sentences, seeing the sentence march off the display to the right is irritating.
To turn on word wrap, go to Settings → Configure Kile → Editor → Appearance. Enable Dynamic word wrap. I also like to choose the Dynamic word wrap indicator to Always on. This puts a tiny wrap icon to the left of any line that is wrapped.
Tried with: Kile 1.2.0 and Ubuntu 12.04.1 LTS