In the last few years, Apple has cracked down on the special characters they allow in App Store descriptions. This is a list of all Unicode characters I’m aware of that they allow as of July 2017. (I suggest copying & pasting these into your own app descriptions.)
‣ (small triangular bullet)
◂ (black left-pointing triangle)
▶︎ (black right-pointing triangle)
◀︎ (black left-pointing triangle)
◆ (black diamond)
√ (square root symbol)
If you’re smart, you’ll use these to improve the readability (especially the scannability) of your copy. Since Apple doesn’t allow things like bold, italics, headings, etc., you can make due with some combination of symbols.