Details about the 5.1 Lollipop update have surfaced very soon after the Android 5.0.1 Lollipop update was made available on all Nexus devices. According to these sources, it is slated to be available by the end of February 2015; and being a 5.1 update rather than 5.0.2, there are several changes that are pretty noteworthy.
Among them are improved battery management, fixes for sudden app closures, general improvement in system stability, and improved RAM management. The improved RAM management especially stands out, seeing as how Android in general demands pretty high amount of RAM. Entry-level and mid-range devices will especially benefit from this, assuming that the improvement is significant enough to be noticeable.
Other noteworthy changes in the 5.1 Lollipop changelog is the re-addition of silent mode after it inexplicably disappeared in the update from 4.4 KitKat. In addition to that, this update should have fixes for other unmentioned issues with Android 5.0 and 5.0.1 Lollipop, such as the persistent “cancel sync” button.
Most of the other changes appear to be fixing existing issues with Android 5.0, like apps crashing and issues with wireless connections. This info is still unconfirmed, but expect Google to get around to announcing the new update as the rumoured release date draws nearer.