You say "I would expect all forums to be accessible from the same place." and others would say the exact opposite. Often when "Lotus" talks about larger "IBM" ideas, like DB2 or WebSphere, people complain that they are interested in "Lotus" information only. So if all the forums were listed in a single spot, I'd expect many of the "Lotus-centric" people in this forum would be complaining about IBM pushing DB2, Tivoli, WebSphere, etc. information at them. So as it stands now, the navigator provides some division of information.
I personally disagree about the Grid computing comment. The navigator on the left is intended to make it easy to find any of the various topics that developerWorks coverers. It is very similar to the navigator on AlphaWorks for emerging technologies: http://www.alphaworks.ibm.com/
So you can see, it's a difficult issue. One that I guess would be best suited with some type of customizable UI where each user could work they wanted.