I am not sure what type Mega Zapdos is, but for now we can assume it retains its original typing. However, it does gain the ability Plasma Gale. Apparently it is merely another version of Drizzle, which causes it to rain on the battlefield for 5 turns.
I don't think the addition of an ability with the same effect as Drizzle is necessary. But there is the capability to spam thunder! And, admittedly, Mega Zapdos looks pretty cool.