Yeah, I do agree. This feels like your average Sheraton, not Luxury Collection. Still Sheraton is my preferred hotel in Sofia. It have the best location for sightseeing, eating, drinking and shopping. I find both Hilton and Radisson to be little off the main path. I also agree that the Sheraton gives you that old-world feeling, but having said that - the lobby and hallways are horrible, gives you that cold communist feeling. I find the bar and the restaurant depressing too, so I don't use them anymore. I always venture out to fullfill my needs. There is also a casino there, which makes the Sheraton kinda tacky, but this is Bulgaria, and casinos are everywhere - even in Radisson and Hilton.
The service is highly inconsistent, sometimes your room won't be serviced for turndown, sometimes you won't receive your wake-up call and I do find the reception staff pretty cold.
The rooms are VERY nice, recently renovated. The bathrooms are very small, but adequate and nice. Huge LCD-television in every room. Free wifi in every room (the Hilton charges € 20 for 24hrs of wifi). Free airport transfers, at scheduled times to and from the airport (ask reception). Extremely comfy beds.
I really hope they will do something with their lobby and bar/restaurant-area! This would easily make Sheraton the only 5-star hotel in Sofia. There are also some high-end shops on the first floor. As Radisson and Hilton gives you that boring business hotel feeling, the Sheraton would be the closest to pure luxury in Sofia - it can be had easily at € 90 per night.