Tag wiki excerpts don't allow any formatting, they're just plain text.
I can see the argument against markup: it would introduce needless distractions. The purpose of the excerpt is to convey information briefly and clearly. If you need newlines or other vertical markup, you're putting too much information. If you need emphasis or fixed-width text, your text isn't clear enough. Links aren't really needed — if you want more information, you can get it from the wiki body.
There's an exception on French Language & Usage: we strive to have bilingual tag excerpts. Unfortunately, the lack of any form of markup means that the two languages have to come one after the other with no good way to separate them. For example, from negation:
Comment exprimer le contraire d'une affirmation ; les formes grammaticales de la négation. / How to express the opposite of a statement; grammatical forms of negation.
At first glance, I just don't see that there's some English there. I find the effect markedly worse in the box on hover which has a narrower column width. It would be nice to have at least a line break between the two languages.
A simple implementation would be to allow paragraph breaks. Dedicated support for bilingual excerpts would be even better: it would push excerpt authors towards including both languages; people wouldn't be tempted to use paragraph breaks for other purposes; the order of the two languages would be consistent.