Sonic has his distinct buffs compared to Brawl, along with a couple of nerfs (most notably his F-air's damage output and his D-smash's range and versatility), but it's the changed physics and hitstun mechanics in Smash4 that are the real source of his dramatically-improved viability.
Combos
exist now, and Sonic's unparalleled mobility allows him to follow up on combos, tech-chases, and juggles even at high %s. That's a HUGE improvement, especially because of Sonic's lack of "traditional" kill-moves (his F-smash, U-smash, and B-air being his only "normal" finishers, and even they don't have all that much power). Being able to more reliably chase opponents off the top using the Spring and improved U-air, or chase enemies off the sides with f-airs and n-airs, is also a considerable buff!
Really, the only things that frustrates me about Sonic right now are:
- He doesn't have Mega Man's up-B behavior that allows Mega Man to save his double-jump for after his up-B. If Sonic had that versatility, his recovery would be nearly unstoppable thanks to the Spinshot, and it'd be even EASIER to juggle-and-chase opponents off the sides and especially the top..
- The occasional painful and humiliating SDs caused by the idiosyncratic behavior of the side-B during recovery and by not being able to easily tell what direction Sonic's facing in midair, leading to embarrassing d-air deaths.
- The meteor on his d-air is nearly impossible to use at all, much less without SDing in the process.
- This might change as new combos and setups are discovered.
Other than that, Sonic's a beast!
