The prologue to episode one (the Great Legendary War itself) is like the Super Sentai equivalent of prologue to Lord of the Rings: The Fellowship of the Ring.
I've been fortunate to catch the first two episodes of Gokaiger, and was greatly impressed with them both; the first appearance of Gokaioh, slashing the Zangyack fleet with its sword within the moon's orbit, brought back fond memories of both Message from Space and Voltron.
As awesome as it is to see so many returning hero teams, via of Gokaiger's keys, I would be more excited to see returning villains in later episodes, especially underused characters like the more note worthy single episode monsters from past series, and the many movie exclusive villains.
Despite that hopeful wish, The Zangyack Empire itself is a pretty sweet group of baddies, and seem to be made up from a wide variety of different alien types, like robots, cyborgs, organic beings, and metallic-skinned, emotional 'daddy-boys' in fancy white and blue, dress wear.