OpenTopic is a totally different product running on a totally different platform using a totally different idea. ;)
OpenTopic uses Java & Oracle in a clustered environment to do some really insane stuff, i.e. millions of pageviews a day. It uses XML & XSLT to perform magic with data... very powerful stuff.
Threads is "just" PHP & MySQL (and whatever else it can be manipulated to run on ;))
(And no, I am not touching the name issue, sorry. []/testimages/icons/wink.gif[/])