Hide or stop advertising the f variable in topic URLs. That is to say, remove it from the links at viewforum and by extension from viewtopic. Example:
(looks similar to YouTube's links, doesn't it:
The URLs still work even if you manually remove the forum id. They still work without it, even for global announcements. All actions continue to work: the post reply, all the topic tools, all the mod tools (move topic etc.). I'd like us to mention and discuss any cases where it is needed to GET the forum id, if there are any. Because if there aren't any issues, then I believe it would be beneficial to hide it.
First, like with YouTube videos, the URL meaning becomes even more obvious, more human readable: viewtopic.php?t=15135 --> "I'm viewing topic #15135".
Secondly, "page rank". The concern of the OP at Why forum variable is in URL? was that when "we move this topic in another forum" its URL changes, and as a result it loses its ranking in the search engines. However, if the topic's URL only depended on the topic ID, it would be easy to transfer the topics to another forum (under the same domain) without worrying about losing their PR.
Currently, the difficulty of transfering forums and posts is a major blocker for upgrading a phpBB installation.
For example, lets say that a phpBB 2.0 owner wants to start fresh with phpBB 3.1 without losing his users and the ranking of his topics. There are ways to transfer the users and maintain their e-mails, usernames and their password hashes. But what about the topics? It could be as simple as copying the old content, creating a new topic and then tampering with the topic ID in the database. That way he could save all of his top ranked topics.
Furthermore, we could add a canonical link element to the page header for this format
viewtopic.php?f=xxx&t=15135to point to its canonical