Here are my views:
Aman - Peaceful; the thing I probably most associate with our past and future stays at Amans is quiet (which is why the recent stories of non-buyout events is so concerning)
Belmond - The type of hotel you would see in a Wes Anderson movie; As my wife has commented at a few places, there is almost a feeling of faded opulence at these hotels
Park Hyatt - Contemporary classic (agree with the descriptions above). Solid service, modern decor that feels classic
Four Seasons - Consistent (this really only applies to the city hotels); If you are going somewhere for the first time and you do not have time to do planning, you know what you are going to get at a four seasons hotel
Dorchester - Similar to Belmond, but a step above; old-school glamour
Peninsula - Top service and in-room technology; closest to asian-level service in the US I have found
Taj - Unique Palace hotels
Ritz-Carlton - a stodgy version of the FS (excludes Ritz-Carlton Reserve)
St. Regis - Four Seasons competitor with generally more modern design
Rosewood - While we have had great stays here, I am not sure if they really have a single selling point. Just so different between Mayakoba, New York and London and while you could make the same case for FS between locations, the Rosewood portfolio is much smaller