Street Fighter X Tekken doesn't play like Street Fighter IV and it certainly doesn't play like Tekken. To be honest, it plays more like a marriage between Street Fighter IV and the Vs. series. It's a faster paced, combo-heavy fighter -- everyone has very quick recovery times and some fighter's traditional moves have been changed to the more newbie-friendly quarter-circle movement. Oddly, Chun-Li's swapped out charges for quarter-circles for most of her moves, while another charge character, Guile, hasn't had his move set changed at all.
Each character their signature moves and comes equipped with a standard four hit combo that ends with a launcher, which summons their teammate in to continue the juggle. Because of this, the game can feel like a button-masher at first glance, but there's plenty of depth there for those who look to find it. Already on the show floor, after just three days of having the game out there, I've seen some ridiculous double digit chains.
Perhaps the biggest deviation from the traditional Vs. set-up is that you don't need to defeat all of your opponents to win. In fact, you only need to best one of your opponent's fighters. As such, each fighter has a lot more life than usual and attacks do a bit less damage, making for longer battles. During the majority of my play time, my battles would last until the timer ran out, but I'd chalk that up more to my unfamiliarity with the game than anything else.
EX moves are carried over from Street Fighter IV, but instead of getting four gauges to fill, there is only three here. Aside from the usual move buffs (which are largely the same, except for some faster-moving projectiles), when knocked to the ground you can tag in your teammate using a notch of the EX gauge. It's pretty handy when you're getting your clock cleaned and just need to get a particular fighter out of there.
As for Ultras, they've been changed to the Vs. model: a quarter-circle and all three punches or kicks will unleash your character's ultimate move. These are as flashy and devastating as you'd expect.
The stations set up at Capcom's booth show a total of 16 playable fighters, two of which are still hidden. Capcom wouldn't comment on whether this would be the total number of fighters in the final game, but it would appear at least one new fighter is teased in the E3 trailer below.
The short of it: I absolutely loved Street Fighter X Tekken. Having a base knowledge of Street Fighter definitely helped me out, but even those characters have been tweaked just enough to feel new and exciting -- your favorite character more than likely has a few new tricks to discover. Then you add in the Tekken characters, who retain their most recognizable moves from their respective games but have been expertly adapted to the Street Fighter/Vs. play style, and you have what is easily 2012's most exciting fighting game. No joke: I absolutely cannot wait to get my hands on the final game.