Exactly
Multiple waveforms including pulse width modulation. You could even combine waves.
Envelopes, essential for mimicking real instruments as a piano doesn't start and stop abruptly, there's an attack and release at the very lease.
Multi-mode analogue filter, which was flawed and they knew it but there wasn't time to fix it. But it was better than nothing.
Ring modulation (an effect famously used to create the Dalek and Cybermen voice sounds).
To get a keyboard with such synth features in 1982 would have cost you quite a bit. Okay, it was a bit rough and there's crosstalk and bugs (the 6581 never fully closes its envelope generator) but you have to remember that it would have been heard through a TV set.
It's influence on some people is indicated by the fact you could get the sound chip in a MIDI module (I have one myself, quite a rare item), check out the owner list, you might recognise a few of them (especially Depeche Mode):
http://en.wikipedia.org/wiki/Elektron_SidStation#Bands_that_use_SidStations
But of course, some ignorant comparer or specs will point out the BBC Micro has 4 sound channel compared to the C64 SID's 3 channels :) completely ignoring the fact that the beeb can only produce boring tones.