NOTE: this is a direct dump of https://wiki.blender.org/index.php/Dev:Source/Development/Todo/Render (without cherrypicking the valid items)
Render & UI
- While rendering, manipulating images (like reload, resizing generated) can still crash Blender. Better image "owner" locking is required for 2.5 (T28477)
- While rendering, several UI changes can cause crashes (T24758)
- Rendering can't really use normal scene data if ui is interactive without causing all kinds of trouble, for examples and related issues see the item above this one.. and the reports (T26535)
- Renaming scene leads to missing rendered result in image editor and compositor (see (T30222))
- Rendering with File Output nodes could produce unwanted output images, see (T38605)
- Shaded draw mode (using render code for vertex colors) is most certainly not coming back
- Color Management can be communicated better, like when sampling colors in Image Window it confuses (could indicate such). Color Management also wasn't added backward compatible, disabling it gives dark results.
- Compositor "file out" node doesn't work for FSA, and as color management troubles too.
- FSA render uses temp files with names preventing other Blender instances to render too. (T28314)
- Texture nodes have been implemented by recursively (backwards) calling parent nodes, this gives issues for viewers, previews and threads in general.We will try to keep it stable, but the functionality for this is going to replaced with a better shader system in 2.6. (compositor and material nodes call nodes forward, sorted on dependency. Each node is only called once this way).
- Render display in Image window, for a window previously showing UV editor, doesn't restore correctly. (T26828). Code for this is too much spread around and fuzzy, needs full revision.
- Object transform is not possible during rendering T37019
- Don't render and warn when a driver or pre-render script can't be executed, a texture is missing, physics are not baked. Needed e.g. when renderfarms have scripts blocked.
- Grouping texture nodes affects on data range T45696
- add option to smooth vertex colors as a post process after baking T35066.
- Baking to a low-res textures might miss pixels which lies on the UV island boundary T37681
- Vertex baking might cause AO artifacts T44178
- Support dupligroups T43553
- Modifier produces non deterministic effects T42015
- Update Blender side baking to support different topologies needed for adaptive subdivision T49405
Render Engine (Freestyle)
- NGons creating unwanted lines in Freestyle T40163
Render Engine (Eevee)
- Linear motion blur is not sufficient to express very fast rotating motions. Non-linear with bezier interpolation should do the trick, or maybe substep decomposition. T51480