Android 5 memory usage for Nexus 5
Android, just like Linux variants, use as much memory as they think they need for optimal operation. Because of this, more stuff may be preloaded, so apps load faster or the interface is more fluent. So, if your memory is almost full, that doesn't neccesarily mean that you don't have any memory left.

On the other hand, Android L uses a lot of animations, so I can imagine it using more memory than before. Nothing to worry about however.