As soon as “The Big Bang Theory” introduced an actual romance between Leonard (Johnny Galecki) and Penny (Kaley Cuoco-Sweeting) there was a concern the show would fall victim to “Moonlighting” syndrome (or “Cheers,” if you prefer), where putting two principal characters together — and breaking them up — risks overwhelming the show. By that measure, Thursday’s season finale (and SPOILER ALERT if you haven’t watched) marked a testament to just how accomplished this series has become, not only getting laughs from all seven regular players and recurring guest Kevin Sussman, but incorporating a poignance and even sweetness that has become one of the CBS mega-hit’s hallmarks.

Here, much of those latter elements came from Jim Parsons as the childlike Sheldon, who — as he states repeatedly throughout the episode — hates the idea of change. So Leonard’s announcement that he’s engaged to Penny and wants to live with her puzzles his scientifically minded roommate, who initially allows that Leonard’s bride-to-be can stay with them “one day a week for a trial period.”

Parsons’ talent (and here’s an early plug for his upcoming performance in HBO’s “The Normal Heart”) has frequently helped “Big Bang” resonate beyond its comedic moorings, and Leonard’s apprehension about letting him venture off on his own underscored a string within that relationship the producers have become increasingly adept at plucking.

Of course, Leonard and Penny’s engagement is apt to drag on a while, but the idea that change is part of the show has already been established. Indeed, this is a very different series than the good one that premiered on CBS in 2007, having added Mayim Bialik and Melissa Rauch as regulars, introduced the notion of balancing fanboy pursuits with marriage and given the women virtually as much due as the central quartet of guys.

Like anything that’s both inordinately successful and hangs around a long time (she helpfully intimates as much), it’s easy to take “The Big Bang Theory” for granted. Still, one could see just how much the show means to CBS by watching the finale, based strictly on all the promos for other programs (including new summer and fall series) the network crammed into it.

On his season-ending vanity card, exec producer Chuck Lorre cited the show’s humble desire to repay viewers for their time and attention with laughter. But at its best, “Big Bang” delivers a bit more than that — something that’s consistently, to use Lorre’s analogy, a pretty good bang for the buck.