So, after installing some additional Stuff Packs, my sims' interactions were severely curtailed; the options would be limited and interactions would often drop out of the queue shortly after starting.
Tonight, I sat down for some 50/50 loving and, after many tries, finally narrowed it down to one (which shall remain nameless)—perversely, one the newest hacks I had (I thought new ones would be fairly safe), dating from September 2008.
So, had a look for a new version, found and installed it, and hey presto, it works! While I was at it, I also updated the other related hacks from the director's cut package (though not the ones that are only available for download separately), gave that a quick test and the interaction problem hadn't reappeared. So that's good.