How do you use search?
Gimp Chat Search
Do you use any of these links?
Gimp Chat Tutorial Index
Gimp Tips and Tricks
Essential GIMP Tutorials
Do you use a tutorial bookmark folder?
Have you thought about compiling such a list for yourself or one you could share with others?
Just curious!

There are currently 480+ tutorials; I'm guessing at least 65-70% have been posted by infrequent or dormant members.

I Agree!

what about making tags available to a topic, like what's used at the plugin registry and gimpscripts? If the forum software won't support that, another approach might be adding descriptors in the thread title. Something like:
Letter Drop Tutorial [Animation][Text Effect]

The 65-70% part will make more sense if you scan the Gimp Chat Index by Author.
Categories are subjective, keywords are better; I like a tag cloud myself.
When you say thumbnails do you mean in the tutorial thread list or in the Gimp Chat Index?
Can you give a few example of 'the best website', I'd like to see how they work?
Keep in mind this is a forum not a blog or website.

Personally i hate nested boards and sub categories i would not like the change

A example of a forum with far too much categories, all containing several"child board" each containing other nested "Child Boards " is http://www.fractalforums.com , i hope GC would not do something alike, browse that forum for me is a pain, also because new post are not visible if are done on "child board" so each time i have to explore all the labyrinth.

for what you wish is much better learn to use search

And as far as improvements would be sufficient allow to "tag" the new topic :

using your example of Tutorial i will found useful Tutorials tagged as you suggested ( GIMP basics, Quick Tips, Animations, Text Effects, Special Effects, Photo Effects, Design, Uncategorized/Other, etc.) that for sure will make search easier...BUT i will found horrible them not as tags but as nested boards.

Also because many of that categories often overlap, not a problem if are "tags" because you may use several tags so the same tut may be tagged " Quick Tips" "Animations" "Text Effects" "Special Effects" " Design"
BUT if instead would be new nested boards ...where you will post a quick tutorial about animated , special logo effect ? ... under "Quick Tips" ,under "Animations" under "Text Effects" or , "Special Effects" or under" Design" ...you will be forced to chose 1

And then to search quickly you will need a magick crystal ball telling you which of the 5 possible adapt boards was chosen

I already have similar nightmares when browsing fractalforums please, please don't do the same here on GC

We are certainly open to better ways of indexing the tutorials and there are some limitations when scanning the master index or using the built-in search feature. Post tags could be a useful addition. We do ask that members who submit tutorials include a final image of the result but we can't always enforce those rules.

Some people don't actually add the content here, rather they post an off-site link to a tutorial (or an image), which usually ends up dead, or moved, and eventually has to be deleted.

Believe me, we're listening to suggestions and taking them all seriously. Thanks for expressing yourselves. Hopefully, more will do the same.

IMO, I think post tags added to the subject line would be the fastest and easiest solution. I agree that it is very difficult to find the tutorials you might be hunting for, so I will be happy with any solution that will make it easier.

It would be nice if tags may be added not only by the tutorial authors but also by registered users