Why is my interface now wlp2s0 instead of wlan0?

A new naming scheme has been introduced, to solve problems that arose from the old (eth0, wlan0) naming standards.

Here is a short introduction and explanation of the concept: http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/

Basically, the first two letter describe the type of interface. 'wl' for wlan, 'en' for ethernet. The following code is often a description of the physical placement of the device in your computer - p2 is likely PCI bus 2, and s0 is likely slot 0.

As kyodake pointed out, this new standard was introduced when Ubuntu moved to systemd.