So, the most ubiquitous decorations are the adventsstjärnor (advent stars) that decorate almost every window.
The stars light up the darkness (which begins around 3PM now in Uppsala) and add a lot of cozy feeling.
The next most common decoration is the adventsljusstake (advent candle holders) that appear everywhere. They're often electric these days, and frequently LED, but there are definitely some with real candles.
Of course the most common decorations are light, because it really gets so dark, especially when there isn't snow yet. Stay tuned for a post about St. Lucia - a true celebration of light!