I upgraded from QGIS 2.8.1 to 2.18.6 and all seems very nice.
One problem I have is with display. Some, though not all, of my line and point layers won't display unless I zoom in. All my polygon layers display at all scales.
The affected layers do not have Scale Dependent Visibility enabled for the layer. So I'm not sure why this behavior is occurring.
I'm having to zoom way in to see them -- so much so that it is not practical.
UPDATE: I've narrowed down the problem, but haven't found a solution. It's one of the layers. When I turn on labeling for that layer, all features in that layer suddenly disappear, and most (but not all) of my other vector layers also disappear. If I disable display of the offending layer, display of all other layers is restored. This issue does not occur with 2.8.1, but occurs consistently in 2.18.6.