Major bugs:
Bug 375503 - MP4 render doesn't play in any program (Semi-solved)
Bug 377549 - Unable to render animation: "No such file or directory" in log file (OSX, with comment from dmitry, but it's marked as unreproducable in T5753)
Bug 376352 - Unable to render animation with error 'no such file or directory' (Windows)
Bug 371553 - Render issues... or "No such file or directory" (Linux)
Bug 371520 - Render Animation encode fails when Render File setting contains folder
https://phabricator.kde.org/T6152 FFMPEG location does not get saved anymore
Bug 380059 - Greyed out UI Elements After Exporting Animation (Krita is freezing on OSX)
Bug 379365 - FFMpeg failed to convert (reopened)

Minor bugs:
Bug 376985 - Render Animation > First frame/ Last frame value are constrained by values from the animation Dock
Bug 373039 - Checkbox to disable automatic backup for RENDER EXPORT

Wishes:

Rendering efficiency:
Bug 376986 - Export frames : make it faster by not re-rendering a frame that does not changes
Bug 377354 - Do NOT delete 'frames in the same naming scheme' (out of frame range) when exporting
Bug 369503 - Render animation encode resume after error,by looking at folder (BB)

Might be resolved with python:
Bug 373360 - render export - export visible layers seperatly
Bug 378288 - Ability to crop the exported animation images/video to a specified x,y,width,height

Other:
Bug 370483 - render export remember location in file (BB)
Bug 364046 - Allow to export animation using commandline (Morevna Project)