The weird thing about DP is that the most useful threads are those I have nothing to contribute to. This is why I think that a bookmark/save/favorite/collect/star feature would work wonders. Now of course, you can follow a thread, but those tend to get lost when people just aren't speaking in that thread anymore. This feature would also be very helpful in the B/S/T forum, quite likely improving the amount of business being made, as it will provide a more nuanced, lower engagement form of interaction, in other words a new step between visitor and hot lead (if you're familiar with sales funnel theory).
You can already watch a thread without replying to it... Just hit the "Watch Thread" link when viewing it.