These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Broken Base: Even with the massive Replacement Scrappy status of the theme, there were plenty of people that seem to have liked Hero Factory more than BIONICLE, and the 2011 sets' move towards more system friendly and smoother elements are made the line more appealing to some LEGO fans. Not to mention that a number of fans also took greater pleasure in HF's infinitely more flexible and easy-to-follow story, whereas trying to keep track of its forerunner's was a chore most of the time.
Cliffhanger Copout: The franchise started following a "you make up the ending"-style storytelling in its second half, which has angered many fans, as they took some of the Cliffhangers as evidence of the story getting better and more continuity-heavy. Perhaps the most infamous case is the first season's Big Bad Von Nebula returning and getting his hands on the Hero Factory's blueprints, a storyline that ended right when it started.
Core Hunter is popular for being a legitimately menacing antagonist who is surprisingly Badass and Dangerously Genre Savvy amongst the villains (and manages to rack up a body count besides). His aesthetic helps.
And, to a lesser extent, Voltix, who looks like he'd be a silly gimmicky villain, but is actually one of the more competent and dangerous antagonists.
Toy-wise, the first series were really just BIONICLE toys with a different name. Then came the 2.0 wave which brought a completely different build system and a major amount of new bricks that then became Hero Factory's staple.
In terms of storytelling, it became a lot easier to take the story seriously after Greg Farshtey (writer of the BIONICLE books and comics) began writing the Hero Factory Secret Mission series. They still stand out from the BIONICLE books in that they're not bogged down with continuity (most are stand-alone stories), but compared to the show they deal with some of the more nuanced factors of the story such as the question of why heroes might turn to villainy and what values a hero has to embody to properly deserve their title.
Hilarious in Hindsight: During Brain Attack, our heroes get a shock when the threat appears on their own planet rather than some distant beast planet. Come the next season, and the monsters top that by rising from beneath the planet.
Some BIONICLE fans (like parts of the BZ Power community) have been annoyed that both Hero Factorys toys and storyline were simpler than those from the franchise it replaced. This mentality has lived strongly for all of the line's run, until news of BIONICLE returning in 2015 surfaced.
Reception for the toys has warmed a little due to the 2011 Hero sets having much more joints and articulation on par with the later BIONICLE sets.
1.0 Hero Factory Heroes were already on par with the previous semester's BIONICLE sets of the same price in regards to articulation points. The villains were also as articulated with the sets of that price from 2009. So, in fact 2.0 made the line have more articulation than BIONICLE's last year, with the cheaper sets introducing knees and elbows.
Rocka is rapidly becoming this, due to the spotlight he's been receiving since his debut in Savage Planet. It didn't help that in 2013 he got two sets in two different waves, while Nex and Stringer were completely absent from the year lineup.
Taken Up to Eleven with Breez 2.0; while the 1.0 set was also rather androgynous, the design of the helmet at least vaguely suggested femininity. The 2.0 set, however, is extremely masculine and very reminiscent of Gali Mistika.
Also reminiscent of Toa Mata Gali from 2001, with the squarish face mask, visor, triangular mouth cavity, and near-identical body proportions, so it's really just following a long-standing tradition in LEGO action figure themes.
Completely justified: They´re all robots, so there is no reason for them to have any physical gender traits; and what constitutes a "female personality" is up to much debate.
Values Dissonance: From the Invasion from Below storyline, the drilling disturbs and scares the Kaiju and Jumpers into attacking. Killing them makes sense if you're about to get attacked, but killing hundreds of babies of an apparently sapient species just because you're too impatient to let the only member of your team who seems to have the situation under control talk things out pushes things quite a bit past moral ambiguity.