This is an older post but I just thought I'd offer my two cents.
What are it's unique advantages and characteristics?
I think the main thing that seperates it from other arts is it's strong emphasis on relaxed movement, proper breathing and dynamic tension. Other arts also have all these things, but Systema really does focus on them alot. If you were to go to a Sambo class and, say, do ground work and then go to Systema class the next day, you would see some similarities. However, the 'feel' is very diffirent.
The other thing that makes Systema unique is it's lack of technique. There are none. It places emphasis on movement over specific technique.
I think Systema has it's greatest value as an internal 'frame' from which your past training can flow. I know that none of this is very specific, but it's an attempt. Basically, I think there are many, many things unique in Systema. I also think it's one of those arts you have to try a few times with someone who knows what theyre doing in order to see that diffirence.
"Seek not to follow in the footsteps of the men of old; seek what they sought."