Right from the start it was clear Google had big plans for Android 5.0 Lollipop. The entire UI had been rethought and some long-awaited features were finally being added. Sure, there were a few gripes over this or that minor feature, but Lollipop looked like a win. Now that we've got the advantage of hindsight, let's look back at Lollipop and see what Google still has to fix in the impending Android 5.1 update.
The Infamous Memory Leak
Google's initial deployment of Android 5.0 seemed to be going swimmingly. Mere days after Nexus devices got their customary updates LG, Nvidia, and Motorola started sending out the first wave of OTAs. Then things got weird and the updates slowed to a crawl, and from what I've been told it was because of memory usage.
Most Android devices still ship with 2GB of RAM, and that's more than enough most of the time, but Lollipop has a particularly nasty memory leak that doesn't show up in system process tracking. Basically, RAM is not being reclaimed properly after process are closed, leading to a memory constrained environment. Background services that you want running (ex. music playback) are mysteriously closed and the home screen redraws frequently. A device like the Nexus 6 with 3GB of RAM seems to be immune from any ill effects, but it's an ongoing issue for many others.
This bug has been reported to Google thousands of times and is one of the most "starred" items in the public Android issue tracker. While Google has marked the defect as minor, it's the sort of thing that can ruin a user experience if a build of Lollipop isn't specifically designed to avoid it. This is probably one of the main reasons the Lollipop rollout has stalled for months. OEMs were waiting on a fix, and now there is one.
Google has listed this bug as "future release," meaning it should be patched in the next major release. That means Android 5.1, as long as it was done in time.