I like the fact that it's possible to work "one thread at a time" and it displays the messages as quickly as I can read them. On lithium, even on a fast desktop, it took so long to display each thread that I needed to open 20 threads in 20 different tabs to ensure the next thread was always ready to read after I'd finished reading the previous thread.
I also like that although I'm primarily a desktop user, the mobile mode works surprisingly well considering the inherent limitations of the phone itself. On lithium I never bothered looking at the forums when travelling, because it was even more excruciatingly slow on mobile and the workround of opening 20 threads in 20 tabs wasn't going to work on mobile. But I'm now able to use the mobile mode when travelling, and aside from device limitations (lack of spare screen space and no keyboard, so hard to type especially in a bumpy DLR train) it works surprisingly well.