Wouldn't the set designs themselves be a hook? After all, that's what the storyline has always been based around. And generally, that's how a toy line operates. The story is meant to supplement the fun inherent to the sets by giving the characters powers, personalities, goals, and backgrounds.
Lets not forget that Bionicle wasn't the most elaborate thing when it first came out too. Sure, Hero Factory has been out for 4 years, but that is because Lego is trying something new, by making it possible for anyone to jump in at anytime. And yes, Bionicle was elaborate from the beginning, but was it really that much?
Put this into perspective:
Evil guy (Makuta) infects a bunch of animals (Rahi) and terrorizes peacful villagers (Tohunga/Matoran + Turaga) so the hero (Takua) needs to summon more heroes (the Toa) to fight the animals (Rahi) in order to save the villagers (Tohunga/Matoran + Turaga) and ultamatley leading to defeating the all powerful evil guy (Makuta). This is where Lego took the great chance to build off the story, and it worked.
Same situation with Hero Factory: (I don't know everything about it but I will try my best)
Oh no! There are a bunch of villians (Equivalent to the Rahi(ish))! We need heros (Equivilant to Takua summoning the Toa)! Heroes get there and defeat the villians and capture them. In which next season they escape again. (Rinse and repeat, soon enough the storyline will get stronger.
And many other things Lego has adopted into Hero Factory has been the Toa Hordika/Savage Planet or the Bohrok Swarms/Attack from Below etc.etc.
I too hated Hero Factroy when it came out. Weak storyline, poor character development, and poor projection of any story whatsoever. Then I realized that Bionicle was not too different, the thing that made it special was they spent time building characters and devloping story. Lego was walking on a razor edge with Bionicle though, because anything that has a very built storyline usually gets people hooked. On the other side of the sword, we have people who find a hard time jumping into the storyline and just buy the constraction toys if they look cool enough.
In the end, Hero Factory and Bionicle arent too different. Lego is just taking much more time for story development and slacking on the peices for Hero Factory.
I will likely not purchase any Hero Factory because I don't feel the need to, it doesn't have me hooked. Which is the fault in Hero Factory, it has no hook. That is why most people hate it now.
I might be speaking from a different perspective than a number of hardcore BIONICLE fans. The storyline and mythology of BIONICLE was not what got me into the theme. It intrigued me, but for me my interest in BIONICLE was a direct progression from my interest in Throwbots, Roboriders, and other Technic sets I had collected before. As a general rule, the quality and functionality of the toy should be a toy's primary selling point.
The story just supplements the play experience by letting kids experience the toy on a more profound level. Thanks to the storyline for themes like BIONICLE and Hero Factory, Lewa isn't just a green robot with a swinging axe, he's a thrill-seeking joker with air powers, a youthful naivete and a strong sense of adventure. Preston Stormer isn't just a white robot with ice weapons and motifs, he's a veteran of countless missions and a leader with a strong sense of responsibility for his teammates.
Most importantly, both of those characters are construction toys that can be disassembled and reassembled into creations of your choosing, and the story supplements that aspect of play by providing a context for your creations to inhabit. The creative nature of this sort of play means that you have to take extreme care with the depth of the storyline. You don't want the adventure you craft for the official characters to get in the way of the characters and adventures that fans create. An overly detailed storyline can become a sort of spider's web reaching out to entangle any stories that aren't expertly crafted to fit in.
I think Hero Factory does a bit better at this than BIONICLE did, though both franchises did a satisfactory job of it. In Hero Factory, you can create your own hero with any powers and appearance you can dream of, and send them on missions either with the main characters or with other characters you create.
In BIONICLE, some factors in the storyline could present creative roadblocks for fans. There is only one Mask of Light, meaning that you can't give a character that mask power unless you come up with a reason why they can have it and where they got it. The Toa Inika didn't go on any new adventures in new locations in between finding the Mask of Life and becoming Toa Mahri, so any new adventures you write for them have to take place on Voya Nui without exception. The island of Mata Nui had only six Toa and only six villages, so if you want to create a Toa with lightning powers, you have to come up with a place outside of Mata Nui where they came from, and they can't interact with the Toa Mata in any meaningful way in between official missions and adventures unless you want to totally rewrite the events of the official story.
Overall, I think the LEGO Group learned some important lessons from their experience with BIONICLE, and it helped them to shape Hero Factory's storyline in certain ways. This doesn't mean that BIONICLE was not a good storyline, just that the LEGO Group didn't have as much experience crafting their own storylines to draw from, and this resulted in some limitations that perhaps ought to have been avoided. And then again, perhaps not. They are different types of story, after all, and perhaps the type of storyline BIONICLE set out to tell made certain limitations inevitable. I think we can all agree that BIONICLE's sacrifices in creative freedom definitely didn't ruin the fun of the sets or the excitement of the story. They just resulted in a unique set of challenges for creators and fans alike.