Masked Man, if you're going to be there until the event closes here's some general things to check out:
Captain Falcon's colors -- is the only new color the gold one? Does the red (Blood Falcon) have any notable differences compared to the previous games?
Is it true that Diddy Kong can only pull one banana? What are his newly animated Up Smash and tilts like, do they possess more range as indicated by the new animations? Does Diddy's Up B still hit walls and cause him to explode off of them and basically die? Can you still determine how far Diddy goes with his Side B by smashing it vs tilting the direction? Also, can you still jump off of people's heads and/or attack them when you've grabbed them with Side B? Anything different about the peanuts?
Is Samus' Up Smash better (as in does it keep opponents in it and can you get people while they are on the ground with it)? How is Samus' new neutral air? How strong is down tilt? Can you try her charge shot and see if anything is noticeably different about it? What about her Up B, is it still strong like the Treehouse people kept talking up about? Is her down air any faster? What about forward air? Also, does she have any new taunts? How far away can she tether to the ledge?
Does DDD's grab still have crazy range or no? How is his Ftilt and Dtilt? Is his back air any different? What about neutral air (the sort of 'belly flop' in the air thing)? Is his neutral B any better or is it the same? How about his Uair and Dair (up air and down air)? Any differences for those? Basically, is anything different about DDD (he's like the least known veteran, I think, in terms of what we know about him) and also, what are DDD's colors? Any new ones?
Some people have noted that Sonic's Side B spindash ends in a kick, is this true? How does the kick work, does it pop people up and away or...? Is Sonic's new Dsmash strong? Or is it more like a get off me Dsmash like Sheik's?
Speaking of Sheik, can you try to shoot her needles in the air close to the ground and see if there's any landing lag for them? In the E3 build she had zero landing lag, practically like Melee where she can shoot needles in the air and land with no lag -- I want to know if they kept this. How about her grenade? Does it still pull in as strongly as it did in the E3 build? OH! How strong is forward air? Is it more like Brawl where it's a weak slap? Also, what are Sheik's colors? Same as Zelda's?
I think that's all I have right now off the top of my head.