Location and comfort. This flat was very comfortable for 3 of us, and would easily be comfortable for 4. Beds were comfortable, lighting good, kitchen small but useful. Common area with tv, wifi, dining table all nice. Walk to everything - Trastevere, Campo de Fiori, even the Colosseum. Only issue was noisy road (even with good windows, you hear traffic). Safe house, safe area, near transportation. Lovely hosts.