Im sure Kirby isnt in the demo because he takes up a lot of room. Note the characters not on the demo seem to have unique mechanics that take up more room (Trainer's 3 in 1, Wario's fart buildup, Ice Climbers duo, Zelda transform?). The no Zelda is quite telling but thats for another thread! Kirby fits in that group with his copy ability...
Anyway, anyone who has paid close attention to the Subspace videos will have noticed Kirby has some moveset changes:
http://youtube.com/watch?v=KaMCPPAEX54
Kirby fighting Petey Pirahna. Pay close attention to Kirby from 30-40 seconds. Twice he uses a spinning hammer move in the air. It appears that Kirby's Side B in the air is more like the Ice Climbers' Side B. I will bet that with 2 hits, it is stronger than Melee's overflipping...
http://youtube.com/watch?v=9vmElR_QtSQ
Adventure with Peach. Watch the latter controllable level. At 1:32 and 1:43, look closely and you will see Kirby running then flipping over and spinning his feet to attack while still moving a bit. A new dash attack? Very likely, its the same move Yoyo Kirby uses in Kirby Superstar when running. Already it looks like a much better move than the fireball from Melee.
I am positive Kirby is getting a buff this time. No way a development team who has played Melee 1000s of times will have missed Kirby's impotency.