Okay, firstly, familiarity with the old games helps. SFIV uses SFII as a template and builds outward, so virtually every combo and counter - from Ken's four fierce to Guile's air throw - work as they always did.
After complaints from playtesters, the version shown from GDC (which used 3D collision detection) has been adapted to mimic the 2D hit boxes of the original, so crossover attacks and Dhalsim's stretching kicks feel the same as before. Frills from later games - the air blocking from the Alpha series and the parries from SFIII - are ignored to simplify things. The decision was deliberate, and taken early.
Of the new characters, El Fuerte's the spiritual sibling of Virtua Fighter's El Blaze - apart from sporting masks and tights, the Lucha Libre stylists also share a penchant for running rings around opponents. After a quarter-circle fireball sets him sprinting, Fuerte's got a reported 18 different offensive or defensive options - he's short on combos, but makes up for it in guessing games.
Meanwhile, tubby fanboy Rufus has a belly sporting wobble physics that shame DOA: Beach Volleyball's, but he's surprisingly quick, offering plentiful anti-air options and a spin that sucks opponents in. MMA fighter Abel and SNK-inspired C Viper appear to have been toned down since earlier versions, but remained troublesome enough to throw a few veteran Guile players off their game.
Also announced - but unavailable - is new boss Seth, the CEO of M Bison's weapons division. Silver-tinged and sporting 'body enhancements', he might seem best suited to the more stylised SFIII or the Marvel Vs Capcom series, but judgement is reserved until the final version.
In the meantime, it's more pleasing to speculate on rumours that Dan Hibiki and Fei Long have been spotted amid the game's concept art, suggesting that Capcom have plans for future DLC.