WHAT PARAMETER FOR WHICH USE?
As pointed out earlier, the RT is often not sufficient to describe the required acoustics of a space. Some more parameters are needed, keeping in mind that quite a few parameters have been tentatively developed, but few have so far survived as well as the RT !
Just as a reminder: Those parameters are computed or measured at a given point in the room; achieving a good value at one point may not mean that one will achieve it at each point in the room!
Spaciousness and Envelopment
Spaciousness characterizes the impression of a large and enveloping space.
Envelopment characterizes the acoustic impression of surround to the listener (as opposed to the impression of frontal sounds).
Clarity and Fullness
Clarity C80 characterizes the intensity of the direct sound, compared to the reverberant or reflected sound. It is defined as the difference, in dB, between the sound energy received by a listener in the first 80 ms and the remaining sound energy. The greater the reflected intensity, the more full the room is perceived and the less clear the spoken word or fast or highly articulated sections of music are perceived. Clarity is sacrificed in a “muddy” room, but a lesser degree of clarity enhances slow passages of music from the romantic era, where one prefers a global sound impression over separate pulpit contributions. To achieve greater clarity or definition, the entire audience should be close to the stage and have an unobstructed view. This can be accomplished by placing performers on a raised stage, and perhaps a raked stage, and by placing the audience on a sloped floor or in balconies.
Assuming an exponential sound decay in the room, clarity, in dB, can be estimated from the reverberation time using the formula 
Loudness characterizes the average power detected by a listener. A longer reverb time will contribute to the loudness of the hall.
Strength, given the symbol G, at a given point of a room is defined as the difference between the sound power level Lw of the source and the sound pressure level Lp:
Using the room parameters, this can be expressed as