One thing that I don't like about this game and the original Street Fighter 2 is that everybody turns into M. Bison when they get hit by a fire attack, i.e. yoga fire. It should've been removed in the remixed version, but I guess it was kept for nostalgic reasons.
It has added moves and stuff. It plays quite similar, and if you want an exact replica you can always choose to play the classic version. Gouki is a FKN bastard.
It uses Super Street Fighter 2 : Turbo, as its base
you can choose to play with the HD art, or the Original art
you can also choose to play with the "Remix Mode" gameplay, or original
"Remix Mode" , is basically a mode where an Ex-Tourney player (David Sirlin?) was chosen to basically balance the game as he saw fit, from a professional standpoint... so most characters' normals, specials, and supers have been adjusted, revamped, removed, or buffed / nerfed, or given a new / easier command input to perform them.
For example, Guile's roundhouse flashkick now goes heavily forward and slightly up, giving him the ability to retaliate against fireballers at medium range (a tool which he never had before), and his upside down kick (a command normal) is now easier to perform, and now counts as an overhead strike (which must be blocked standing or the opponent will be hit).
there's also a nifty "view hitbox" option in training mode, to help with all of those nuances