The game had a rocky reveal history: Generations was originally announced for PS3/Xbox 360 only. The 3DS version's existence was first hinted at in a GameStop listing on May 7, and confirmed in a Sega-Sammy presentation on May 17, along with the PC version. However, both the 3DS and PC listings were removed from the same presentation a few days later for unknown reasons. Finally, the June 2011 issue of Nintendo Power featured classic Sonic on its cover...and the 3DS version was finally confirmed.
The build shown in Nintendo Power seems to be identical to the one playable at E3 2011, featuring both acts of Green Hill Zone and the boss battle against Big Arm. Nintendo Power also included a few screenshots of the first Special Stage.
General Differences
The E3 build contained the infamous "uncurling" mechanic carried over from the DS games: Classic Sonic uncurled after flying off ramps, even if he was rolling prior to being launched. This was also the case in the console demos and the "feature" caused enough outcry that the behavior was changed in both versions by August.
The title screen featured a different design around the "Press START" graphic, and lacked the unique sound effect from the final.
Level selection in the demo was via a simple menu. The top screen showed the background of the Zone and large artwork of the Sonic selected, while the bottom screen showed the list of levels. The final game uses an adaptation of the White Space concept seen in the console versions.
Demo versions did not include the hint feature (which was represented by a ? icon on the bottom screen).
The "Start" and "Goal" text were smaller, not in caps, and unpunctuated, except for the "START!" text in Special Stages.
Classic Sonic is a different shade of blue and uses a running emulation similar to Sonic the Hedgehog 4: Episode I, with a flat spiral effect.
Both Sonics lack drop shadows.
Green Hill Zone, Act 1
E3 2011
Final
The structure in the background was completely different and much simpler.
A palm tree on the left was removed.
E3 2011
Final
A palm tree was added to the left of the platform, while the sunflower and rock near the purple flowers were removed.
The Motobug gained blue irises for the final.
The sky uses a simpler texture with far fewer clouds, and the mountains in the background are few and far between.
E3 2011
Final
The bridge posts use murkier textures and erroneously float above the ground. The final posts added more defined wood and also added a bit of green, in order to connect them to the ground.
The water texture is plainer and doesn't feature the ripples from the final.
Again, the mountains in the background are configured differently - there are far fewer of them.
E3 2011
Final
The Results screen had nothing but a single palm trunk and sunflower. By the final, the area was beautified with tons of foliage and more land.
Classic Sonic's animation upon getting an A-rank seems a bit different, and he ends looking to his right. In the final, he looks at the camera.
Green Hill Zone, Act 2
Sonic starts from a standstill in the demo. In the final game, all Modern levels except Casino Night begin with Sonic already running.
All button prompts are missing.
Sonic is missing most of his voice clips, especially noticeable when he boosts or performs the homing attack.
At this point, Sonic could wall-jump off of any wall, without any button prompts, like in Sonic Colors. This is not the case in the final version.
The Homing Attack is a little finicky here, and it will sometimes miss enemies that are right in front of Sonic.
The Boost meter doesn't make any sound when it enters its temporary blue mode. In addition, while the meter is in blue mode, it doesn't show the silhouette of the yellow bar behind it.
At 1:10, crumbling totem poles were added to the final, which Sonic can attack and use as stepping-stones to higher areas. The same applies for the area right before the scripted giant totem pole, in which case they act as obstacles instead.
It's not necessary to boost away from the giant totem pole in the demo. Attempting to replicate this in the final game will result in a horrifying death instead.
E3 2011
Final
The results screen is clearly unfinished - there are no decorations whatsoever unlike the Classic version.
Sonic's model pops into the screen after the geometry loads, and his animation for obtaining an S-rank is glitchy; in the middle of his jump, he cuts to his lying pose.
Special Stage
No playable version of the Special Stage existed at this point, but a few screenshots appeared in Nintendo Power, and there are several differences from the final.
Top Screen
The sea floor (the background texture) is a lot lower compared to the tube here than in the final.
The green crystalline supports outside the stage are missing- instead the tube seems to distort the background a bit more than in the final version.
The stage's time limit (red) is irregular here; it says "1:24.11". All Special Stages in the final game have a time limit of 1:30.00, but the strange number here hints that the red timer may have served a different purpose.
The Boost Gauge is bare, missing the "Y: Boost" graphic, and also lacking the timer that keeps track of how long a single boost lasts.
Touch Screen
The blue upper graphic is slightly misplaced, as it's a bit too high here.
The Sonic and Emerald icons on the progress bar didn't even move at this point - they just stayed in one place as placeholders.
The blue bar and star behind the Sonic outline is missing.
The Blue and Yellow Chaos Emeralds were swapped for the final release.
Big Arm
The biggest change here is the music: the E3 build uses a light remix of The Doomsday Zone from Sonic & Knuckles, while the final game uses a heavy rock remix of the original boss' theme.
The boss lacks its health meter in this version, forcing the player to keep track of any hits landed.
When Big Arm smashes the platform, Sonic is stunned by the impact for about a second, represented by his "balancing" pose. In the final game, this only happens if Sonic is very close to the point of impact, otherwise there is no effect.
