The room is nice: there's a bathroom with a shower and there's Wi-Fi, air conditioning, as well as a fridge (this last is in the kitchen section of the apartment, not the room). Breakfast (upstairs) is included.
Pretty standard stuff, no? So why the five-star ratings?
Firstly, because of where the apartment is: when I learned out of my window and turned my head to the right, I could see Castel Sant'Angelo, less than 100 metres away (Note: there is more than one room so I can't guarantee you'll have the same view). This isn't just a great view in itself, it means that you're right in the heart of Rome and that you have a fifteen-minute walk to the Vatican, and a slightly longer, but very doable, walk to the Colosseum, the Forum, etc.
The second reason is Alessandro himself.
He was extremely pleasant and helpful, always happy to supply information about where to go, where to eat, and how to get there; or even just to have a chat.
Rome is a wonderful city and this is a great base to explore it from, and the place I'll try to book when I come back.