The author is referring to the fact that the taskbar/dock in Unity doesn't offer the same affordances as the old Gnome2 taskbar. For example, in Ubuntu 11.04, there was no way to open a new terminal from the dock. You had to click on the terminal icon, then click File->New Terminal. As someone who routinely opens a number of terminals for various tasks, that one regression was a source of significant slowdowns.
In addition, Unity is slow on any system that doesn't have a separate graphics card or the latest generation of integrated graphics. There doesn't appear to be a way to turn off the graphical effects to speed it up. Combine that with the usability regressions, and Unity was a definite no-go for me.
That's not to say that I "hate" Unity or will never use it as my desktop environment. I think Unity has a lot of potential. Unfortunately, like all too much software out there, it was released before it had been fully finished. I look forward to trying Unity again when the next Ubuntu LTS (12.04, I believe) is released. Hopefully it'll be far more usable and "finished" by then.