SF2HD is newly re-balanced from the original, and features "tons of changes" according to Sirlin. The HD remix will also now include support for widescreen 16:9, and feature remixed music. And, as originally hoped, included in the package will be an arcade-perfect version of Super Street Fighter II Turbo, with no changes made.
In order to achieve the best online fighting experience possible, Capcom is attempting to duplicate (rather than license) the networking technology of GGPO, which attempts to streamline online gameplay in fighting games for the least lag possible. Sirlin himself is a game designer currently hard at work balancing the gameplay of SF2HD, so when he says it's looking great, he would know.