Personally, I really enjoy the whole ancient galactic war between the two factions story a lot. I've always enjoyed that sort of thing, though, and I can definitely see why it gets very boring and repetitive, very quickly for many people. Obviously, they advertised Onslaught as this big WAR expansion, finally getting your hands on the enemy side's necks and destroying them. But it was literally just a few quests on two new planets, which were cool, and a FP that consisted of... well, nothing. I thought it would be more like taking back Korriban after it's invasion by Jedi, or invading Tython, with the DDay-esque battle and carnage, but I was wrong.
Obviously this story will continue, but who knows how long that'll take, and what is there to do in-between now and then? Like everyone else says, the same ole... grind FPs, dailies, maybe an OP or two, some PvP.. the same, until we get some more story stuff, which hopefully takes longer than 2 hours to complete. I honestly really liked being able to rejoin the Empire or stay with the Alliance, and of course, imo, Malgus is sick as always, but yeah.. it was kind of a let down.