There is a structural problem with megathreads as well. Let's say for instance, that you remember seeing a photo of a really nice roth posted and later you can't remember who posted it. Imagine that you even forgot the clonal name, but you really want to find a copy of that photo because you think you found a similiar plant for sale. So you come to Slippertalk and perform a search for "rothschildianum."
If the forum is set up so everyone can create their own thread about roth, you may have to scroll through ten pages of search results in order to find your photo. Maybe the title of the thread or the name of the user who posted it jogs your memory and makes it easier, maybe it doesn't. This is the way the forum operates now. It can get tedious to search sometimes, but that is what you get for being forgetful. : )
However, if the forum is set up so that every roth photo is now posted at the end of a megathread, you now have to look through all the pages of the thread. This could be 30 pages, 40 pages or even 100's of pages. If you are looking through the thread for a photo, you have to wait for all the images to load on every page, which on dialup could take quite a while.
Also, if you post a photo of a roth in the Megathread and a few months go by and many more people post in the thread after you, your photo is now in the middle of the thread somewhere. Now say that you want to point someone to your photo. You can't just go to the end of the thread to see your photo anymore. In the past, you would search the forum and find the thread where you posted the photo and link that person directly to the thread. If you have megathreads everywhere, you now have to search the forum and potentially dig through the megathread for a few pages, then link the person directly to that page.
Also, when you search for "roth culture" on the forum now, the site searches all the thread titles AND body content for that phrase. So you might pull up a thread that is titled "Bob's roths" in which Bob talks at length about his technique for growing roths. If you switch to a megathread format, when someone searches for "roth culture", they will see the megathread listed in the search results, but when they click on it they will have to start at page one and click through every page in order to read the culture information.
I really don't see the benefit of having a megathread. They will become huge and potentially messy as older photos are deleted leaving dead image links throughout the thread. Nobody will be able to easily find culture information dispersed in them.
What I would suggest is this. Start an image gallery. I am absolutely sure there are plugins available for vBulletin that allow you to copy photos that have been posted to the forum into a gallery where the photos can be labeled with the grex info. If the photo is deleted by the original poster, it disappears from the gallery with no dead image links. This way, you can have a great visual reference of the plants and also have the freedom to post multiple threads about the same species.
I also like the idea of adding subcategories to the Paph section. When you click on the link into Paphiopedilum culture or photos, you see subcategories like multifloral etc. I would caution against getting too technical with these subdivisions, because you could make it very hard for a newbie (like me) to find where to post a thread if the forum is broken up into "thingamajig subsp. whatsit" categories everywhere. Heck, I don't even know the names of these subcategories, so there would be big learning curve for me while I figured out where to post new threads. I am sure I would not be the only one frustrated with this at first, but eventually I think it would benefit me to learn these things.