It's hard not to love Marco. He's a one man band that manages to keep up a killer product, maintain an active presence in the community and he's innovated on Instapaper in a good way. I use it every day.
2. Day vs. night mode. You can configure it to automatically switch to white text on a black background at night.
3. Performance - even with 100+ articles the app doesn't slow down and downloads new articles in seconds. Switching between articles and scrolling long articles is liquid smooth.
Overall, you can tell just by using it that a ton of thought was put into the UI.