Just to clarify what I mean, I initially entered Imperial Assault somewhat cautiously in early 2016, right around the same time Rebellion came out. The base game seemed cool, and I liked the idea of skirmish (ironically, I've since played almost exclusively campaign).
To me, the main draw were the minis, really. I first got the Core Box and all of its associated packs, then went and got Twin Shadows and Hoth (also with their associated packs). I gradually started picking up the other packs (the bantha, smuggler, etc) and I got The Bespin Gambit when it was first announced.
If anything, from those early months, I remember being really overwhelmed. The idea of "waves" was something that I latched onto pretty early on, since it seemed to dictate what I should or shouldn't buy at any given time, but even so there was so much to learn, so many packs that I needed to place in their respective waves.
Nothing really stuck out to me, though. It was all classic Star Wars, but I think that was kind of what made it not particularly stand out to me. It's not that I disliked it, but since I hadn't started even playing the game yet, it just kinda seemed like Rebellion had a lot more driving narrative force (which I still don't necessarily disagree with, from a certain point of view).
Obviously the minis were cool (especially the AT-STs!) but I really didn't appreciate the heroes for what they were yet (again, it would be several months until I actually played the game).
For some reason, my perspective really changed with the game with the announcement of Wave 7. The inclusion of Obi Wan and Greedo in the game were totally unexpected, but very welcome. Instead of going through the motions and bringing in the obvious characters (like the then unreleased Jedi Luke, for instance) we were getting characters who really didn't fit quite into any of the stories yet- and that was totally okay with me!
What really got me was the Grand Inquisitor, though. It would be about half a year before I even tried to begin Rebels at that point, but I'd seen enough promotional material to know that he was the show's big bad (at least for a while). Even not being a fan of Rebels then, the concept of the Grand Inquisitor joining the game really blew the roof off of this game's potential for me. Again, instead of just going through the motions ("we did Hoth, now let's do Bespin, then Endor") Wave 7 opened up the game to the new canon, which at that time I was still pretty unfamiliar with, though eager to learn about.
I think that Jabba's Realm was an exciting announcement, but it didn't have the pizzazz that Wave 7 did. Luckily, Waves 9 and 10 both continued and built on my enthusiasm for Wave 7. More Rebels characters, stuff from the comics, some light prequel sprinklings, and some iconic stuff like the Emperor and Jawas to temper the more out of the box elements.
So yeah, for me, Wave 7 was what really defined Imperial Assault for me. But I'm curious what others have to say. Obviously, if I'd started playing earlier, maybe something like Return to Hoth would've been equally as surprising (especially since it wouldn't have been lumped in the large pile of components that were available when I started playing). And I'm sure for many, stuff like Wave 7 is actually pretty polarizing, especially with the precedent it set for stuff like Wave 10. Looking forward to hearing your thoughts!