In the beta screenshots of Sonic there are some differences: title screen, the mispelled word “RING” instead of “RINGS”, Sonic that loses rings in a strange way, Sonic punching the air at the end of the level, strange things in Starlight Zone in the background and UFOs in Marble Zone. Sonic Team originally planned 5 acts a stage..with 15 stages.
Development for Sonic the Hedgehog began in April 1990, after Sega ordered its AM-8 team to develop a game featuring a mascot for the company. After choosing a hedgehog as the main character, the 15-man group changed its name to Sonic Team and started working on Sonic the Hedgehog. The main minds behind the game were character designer Naoto Ohshima, game programmer Yuji Naka and designer Hirokazu Yasuhara.
The game was originally intended to feature a sound test menu, with animated graphics based around Sonic break-dancing to the music of a “Sonic Band” consisting of Sharps Chicken (guitar), Max Monkey, (guitar), Mach Rabbit (drums), and Vector the Crocodile (keyboard/synth); Vector was later re-designed and re-used for the games Knuckles’ Chaotix and Sonic Heroes. The development schedule meant that the feature had to be scrapped, and Yuji Naka decided to replace the test with the “SEGA!” chant used in TV advertisements, which took up 1/8 of the 4-megabit cartridge. A text-only sound test option remained in the final game’s level select cheat menu.
[Some info from Wikipedia]
Also, Kondensaattori100 noticed some beta differences in the video below:
1.Green Hill Zone boss weapon
Thanks to Zero 7, FullMetalMC, Assjerk, Parker Verboom, Youlute and Rod_Wod for the contributions!