Has Lt been tested in high traffic environments and with huge watch lists? I am an admin in two high traffic project and have a watch list with 3500 entries. The "new messages" page is useless under these conditions. Furthermore LT is unusable in high traffic environments: Try finding the three new messages in a tree of 400 or more contributions. Or worse, the 26 new messages in a 1600 entries tree. And yes discussions like that happen, my fav example are discussions over notability criteria on deWP. On traditional wiki-style talk pages I can call a diff over 26, 65 or in extreme cases 220 versions and scroll down. I then have to read the new stuff in the small font or zoom my monitor display, but it works. Perfectly fine, once I understood how things work.
Topic on Talk:LiquidThreads 3.0/Design
Yes, diffs are useful. Their problem is that you don't easily see the context. Perhaps we need LQT to allow collapsing of already read messages in a thread, beside highlighting of unread messages.
We'll be working on New Messages in a design meeting in the coming days.