With a locked bootloader you can only use Stock based roms – so no Kit Kat custom roms on a LB until Sony release 4.4 themselves.
These Roms will typically have the latest Sony media apps (camera, Walkman, video etc.) and use the Honami (Xperia Z) theme. They are often de-bloated and tweaked so their free RAM and out of memory settings improve on normal performance. Other features may include an expanded power menu and long press volume key to change track when listening to music.
Existenz is the most fully featured custom rom you can install without unlocking your bootloader.
An alternative is to install a debloated ROM and use the Xposed framework to get some of the goodies from Kit Kat and other AOSP based roms. This is what I did before CM was developed for the SP.