I can choose a white background; and i can change the transparency of the overall note including the  text. However, it is in some cases quite useful to have a transparent background but no transparent text, e.g.:
- I want to annotate a figure. (sometimes)
- I want to add text between lines with minimal covering of the text by the sticky note. (quite often!)
- I want to add decent subtle notes on backgrounds of different colour. (seldom)
Background transparency would thus be a nice feature!

(It's called Haftnotiz in German and I suppose that translates to Sticky Note. However, I might be wrong.)