I confess I never warmed to Cassie the way I was obviously supposed to (My dislike of Byrnes WW run to one side).
She was a writers "pet" creation, and whilst I can love those to death (cf Claremont's Doug Ramsey) she just came across as really, really... generic, and a weird attempt to recreate a character that they already had available in the form of Vanessa Kapatelis.
As you say, her powerset was the standard flying brick, and her personality was very thin compared to Vanessa, who was by no means perfect (and would have made a poor WG I think), but had a personality beyond "Wonder FanGirl" which I never really picked up on from Cassie.