Great list of ideas, here's some more.
Said it before and say it again, undockable menu's would be a huge workflow improvement when you have to repeat certain tasks. Imagine having to add these Labs SOPs a few times :
Also better RMB context sensitive menu popups. Currently the RMB shows me the same options no matter what is selected.
For instance for modeling when edges are selected with RMB click show: edgedivide/edgeslide/etc. With polygon selected RMB > polyextrude/polybevel/ etc. and so on...
Quickly hide and isolate geometry per viewport without having to add unnecessary "visibility" SOPs. Just select an object or some geometry and quickly isolate it in a single viewport, but still have it visible in another.
Better naming of SOPs !! I mean, "Mountain" or "Peak" for normal displacement or translate SOPs, "Creep" for a surface deformer, "Fuse" for a point merge SOP. Why not call it "Point Fuse" at least, so anyone can find it with a simple Tab search looking for a "point" related SOP. Someone at SideFX has a weird sense of humor.
Because of this, as a newbie its often not possible to find a SOP because the naming is just not intuitive. You have to already know the name of a SOP or google it, in the hope you're able to find some info.
I've seen people trying to script a solution for a problem, when there already was an untraceable SOP with a weird name for that.
Disable the possibility to add keyframes where they don't work. Very confusing when you figure out after a lot of fiddling around you're not doing something wrong, it's not working by default.
Like the Vellum Constraint settings for instance, where it looks like one can add an animation curve but only the first frame is read, so it's useless. Or even better, actually make everything animatable.
When setting viewport shading with a radial menu in a quad view, it doesn't seem to matter which view the mousepointer is hovering over. You first have to activate the view with a hotkey (N) before one can set the shading, which in my opinion is a heavily outdated workflow.
Peeps are suggesting adding AI to Houdini, let's make the GUI a little smarter first :-)