Hmm, I've got Leopard and I've categories in the Spotlight menu, if that's what you mean. No problems there. And I've got categories in the System Preferences for Spotlight, no problems there either. Likely it's your computer and your circumstances if you don't have those.
On the other hand, I do believe the system did change a little, since there's no longer several different types of search windows. IIRC in 10.4, choosing to show all results from the Spotlight menu would give a window that resembled an RSS page in Safari. However, there were also other types of windows for searching and creating Smart Folders available from the Finder. I believe they dropped all the different types for the sake of consistency and much less confusion. Now there's just one type of search window (and it doubles as a normal Finder window) regardless of whether search is initiated from from the Spotlight menu, the Finder with Cmd-F or Cmd-Shift-F, or from within a Finder window. It's also the same window for creating a Smart Folder.
You can still organize the listing by category with the Kind column, and focus on just one kind by adding a search criteria if there isn't already one. By default, the first criteria is automatically set to Kind and has "Any" chosen to show everything. Just change the type to instantly switch back and forth between different document categories such as Images, Documents, Movies, PDFs, and so on.
The only other interesting thing about the old RSS style results was being able to organize by time. That's just another search criteria that can easily be adjusted. It's also been addressed by default saved searches (aka Smart Folders) in the "Search" category in the Finder's sidebar, via Finder's preferences. So, any search from the Spotlight menu or Finder can be adjusted for kind (category), time, or practically anything else (other) and saved for easy access from the Finder.
It is a little different from Tiger, but I find it's also a lot more flexible and less confusing to use this way. Also, the consistency keeps Spotlight from appearing as a separate entity that needs special training. Instead, it's just another name for searching in whatever app there's a search field, that happens to transparently enable the possibility of smart folders or saved searches (same thing really, different only by thought).
If the lack of various search windows isn't something you feel are missing, then as I suggested before it's likely your own circumstances and computer. You've probably made some changes I haven't made, and that others also without your problem haven't made. After fiddling as much as you seem to have done trying to get things to act the way it's expected without starting from scratch, it's probably time to give in and start from scratch.
Have you tried a fresh install and testing it out before making any changes from the standard install? By doing that and slowly making the changes a little at a time, you can troubleshoot what changes you are making that is causing the issue.
It sounds like you have a few different things not going as expected. Try to not ball them all up into one problem too quickly. Hopefully, some of it was just a matter of perception or expectations based on how search windows appeared in 10.4 compared to 10.5 instead. Sounds like there are other issues, but it seems like too many different issues are being combined and making an unwieldy problem. It can get excessively challenging to troubleshoot when overly frustrated by such a Frankensteinian monster of a problem. 😉
Good luck in breaking it down. 🙂