No download limit, no activation limit, no time until your download expires.
I'm not even sure you have to activate the games, I think the impulse engine handles that automatically. When I get a new comp, I download impulse, log in and press download on my old games bought through impulse. Download is reasonably fast (500kb/s) and it extracts and installs automatically. It couldn't be smoother.
one activation per Windows install, or something, and even then in some circumstances (IINM non-hardware change) keep the signature file and not re-activate.
there is an activation, but it's pretty tame.
basically, SD does online activation the way it's meant to be done; unobtrusively.