Missing LayerSwitcher in OpenLayers3?
This question was asked on Twitter recently https://twitter.com/RemiBovard/status/525028570780139520
If you follow the answer, at the moment, integrating layer switcher in the core is not the priority but there is an available component at https://github.com/walkermatt/ol3-layerswitcher
You can also take a look on "The book of OpenLayers" samples (by @acanimal) for some custom legend implementations.