Mark V retexturing capabilities. Ability to toggle external textures in real time with "toggle external_textures". Ability to copy a texture to the clipboard with CTRL-C, and paste it with CTRL-V. Even to an image editor. tool_texturepointer tells you about texture in front of you.
Mark V supports an special "HD content models + textures folder" which you can name, type "hdfolder hd" to set c:\quake\hd as your replacement content folder. To switch to a different set, you could do "hdfolder qrp" which would switch to using c:\quake\qrp. Typing hdfolder "" sets it to nothing.