Off the top of my head, here are some ways it could be improved:
Why can't it grey out the options I don't have permission to modify or at least show an icon in a column which indicates read-only?
In sections such as HKEY_CLASSES_ROOT why can't I click on a file extension and have a summary of what process opens that file, what shell extensions are associated with it and so on. Why can't I click on an interface IID and see what DLL or EXE hosts it and also what objects interfaces are by the same binary. Why can't I deregister the DLL from the editor. Separate tools like OLE view are necessary for this but they could be incorporated into regedit.
Why won't Regedit help with all the bullshit introduced with Wow6432 where chunks of the registry are in different places thanks to various 32-bit and 64-bit differences? e.g. maybe let me flip from 32-bit to 64-bit view or show both together in separate panels where I can see the information in one place.
Why can't I live monitor changing values in the registry? Why can't I snapshot the registry and observe what values have changed since the last snapshot?
Why can't I right mouse on a REG_SZ holding a path in the registry and choose "Explore to here"?
Why can't I import / export bits of the tree using the clipboard instead of via files?
Some changes are more modest than others. I'm sure people who spend their life in this tool could think of other ways. The point is that for all the effort has put into the front end of Windows, they're barely lifted a finger to improve the lower level stuff.