The main strength of LQT is also its weakness: potentially you can reply to a five-years old huge thread and bump it to the top of the talk page and to lots of users' Special:NewMessages.
Currently, users won't reply to old discussions because if you read only the page and not the history you don't find easily new messages on old sections, which are less visible; and you are actively discouraged from replying to archived discussions because nobody looks at the archives and to revive a thread you should de-archive it (and you can't).
With LQT, you may even not notice that a thread is archived: try e.g. [[strategy:Thread:Talk:Task force/Wikipedia Quality/Quick update]], where the only hint you have is that "From Task force/Wikipedia Quality/Archive 1" under the title (warning! the talk page is huge – I always get an unresposive script error); and obviously if old threads are just put in some "next page" you are not supposed to move them in a subpage.
Summarizing could help, but not every thread will be summarized.
I think that users should be discouraged from replying threads which: are more old than x; or are e.g. in the third page (assuming 10 threads per page); or have been summarized with a consensual ssummary; or have been closed with some new "thread status" feature.
Please note that due to bug 24814 it's currently impossible to completely archive and protect a talk page. In fact I wrote this as an example of that bug, but it's a separate issue. There's already a discussion on this at http://www.mediawiki.org/wiki/Thread:Talk:LiquidThreads/Redesign/Discussion_status , I think thaat we can continue there and leave this bug here as placeholder (if you wish).