In this sentence:
The exterior of the building wasn't what made it special, though, it was the people inside.
should the second comma be a semicolon, or is it correct just the way it is?
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The venerable US textbook Warriner's English Grammar and Composition (1969) states simply,
"Use a semicolon between independent clauses not joined by and, but, or, nor, for, yet"
Strictly speaking, then, the sentence: "'The exterior of the building wasn't what made it special, though, it was the people inside'" contains what Warriner and others call a "comma splice" and is in error. Changing the second comma to a semicolon would repair the problem, as would adding the word "for" after the second comma.
As commenters have noted, in a more contemporary world than the one depicted in my battered Rime of the Ancient Warriner, this may be more of a stylistic matter than a purely grammatical one. For reasons that sages have given, a full stop seems a bit abrupt, and leaving the sentence as written might convey an easiness of tone that might appeal to all but the most persnickety readers.
If it were my sentence, I'd use the semicolon. If I wanted to be really erudite, I would use a colon, which Warriner prescribes
between independent clauses when the second clause explains or restates the idea of the first
This seems to describe the sentence given here, so I might be inclined to recommend a colon.