there are several guides to this but I'll just run through the most obvious
WDW's Space Mountain is the first Space Mountain built and uses the same basic track type as the Matterhorn but the desing is changed a bit to be more "spacy", still the basic bobsled design remains and it even has two tracks just like the matterhorn, as the first Space Mountain WDW's has the most light leaks
Disneylands space mountain was built differently and was built mostly underground due to space coonstraints, DL's has a different type of track and has a much more thrilling design according to "most" reviewers, DL's Space Mountain features a different seating arrangement and is not bobsled styled due to a different track design and the fact that a single track means you need to take up more people per car. Disneylands Space Mountain is getting reinovated for the 50th and will feature dual sets of Special effects one for the nightime and one for the daytime. When Disneyland Space reopens with the new effects it will still have the same track system however
TDL's Space Mountain is essientaly the same design as Disneylands origional Space Mountain before the reinnovation
Disneyland Paris Space Mountain is totaly different from all the others featuring a different plot and a launched start along with several inversions in the ride, it was recently upgraded with new effects much like DL's becoming Space Mountain Mission II
HKDL's will feature the same track as DL's new Space Mountain also and will feature it's own effects set but it's unknown weather it will feature different effects in both the night and the day the way DL's does
"We all have sparks, imagination! it's how our minds... create creations!"
All I know is I just got back from DLP and Mission II is the best Disney Park rollercoaster I've ever been on! (I have not been to Tokyo however) We rode it over and over and were always impressed! Awesome ride!!!