To hop in here, I think the difference between between the DA:O personality types and the DA2 personality types has, actually, nothing to do with their personality at all.
DA2 characters are easily identified by their tropes/traits, just like DA:O. But the reasons they are like their personality are, essentially, one of the only things you ever talk to them about besides yourself.
Example: Sten is a no-nonsense, completely rigid, tactical taciturn. This because of his up ringing in the Qun. We find this is supported by his belief the Warden is wasting time doing quests such as the Urn, when the Blight should be the primary concern. This is slightly subverted by discussing his preference for sweets, like cookies and cake. Point being, entire discussions are devoted to topics other than the Qun or the friendship/romance with the Warden.
Zevran is a devil-may-care, morally ambiguous character due to being raised I. A ****house and then being sold into slavery to the Crows and trained as an assassin. This is supported by his general demeanor during conversations, by the fact that he bargains for his life to the very target whom he was hired to kill and tales of former exploits as being a Crow where he showed varying degrees of silliness and guile to survive and accomplish his mission. This is subverted by his tale of his Last Mission, where he was shown to be deeply in love with one of his former Crows and who he found was actually punished under false pretenses. Again, there are discussions of his homeland that are completely independent of his upbringing or his time with the Crows and, again, conversations focusing on the Warden's friendship/romance.
Anders is a tormented Mage who's possession by Justice and persecution as a mage makes him melodramatic and oftentimes overly aggressive. He talks of no other topics than the Mage rebellion, his possession by Justice or Hawke. Any variation with the Friend/Rivalry path will only focus on one of these three topics - Justice, Mages or Hawke.
Fenris is a sullen, caustically angry elf who's life of slavery as a living weapon makes him untrusting of others, especially mages. Regardless of his Friendship or Rivalry path, he still offers no conversation options that aren't about either his time as a slave (his master, his transformation, him losing his memory, his sister, etc.), his hatred of mages in general or Hawke.
In DA:O, I feel we had two major differences.
One, different topics of conversation other than the main character other than the 'hot' topics for the NPC. This is huge, as no one wants to talk of nothing but your personal hang-ups EVERY time you go to their house. The only conversation changes we see from most DA2 characters is when they are talking about Hawke... which is a little bit silly.
Two, there are subversions of this personality. Sten likes cookies, despite being a hard asp. Zevran actually cared for someone and picked a fatalistic assignment to end his suffering, despite not appearing to take anything seriously. Leliana is a girl-next-door, only cares about shoes ditz... who is subverted by the truth of being a master assassin bard.
DA2 had small subversions of these roles (mostly in the form of party banter), but the subversion was that characters who are super serious and stoic are actually being funny... and many times the way they express that comedy is by rolling their eyes (metaphorically, of course) or making snide comments... like a sullen, angry person would do, which isn't that much of a personality subversion.
I could keep going with both DA:O characters (Morrigan cares only for power, but not material, petty things - conversations about her past show that she talks about other things, including a subversion of this personality with the story of the locket) or DA2 characters (Merrill is the giggle-squee blood Mage, who is innocent to a fault, except for the fact that she is a malifecarum, only points of discussion are being away from her clan and the mirror and, of course, Hawke) but I think I've made my round-about point.
We know most of what we know about Orlais, Antiva and the Qun from hearing Zevran, Leliana and Sten discuss their homelands, giving details, memories and stories that brought it to life. Yet from Anders, Fenris and Isabella who grew up in the Circle, Tevinter and Rivain, we learn nearly nothing of the cities, their cultures and what defines them. We only know that Isabella has abanonment issues because of things that happened in Rivain, that Fenris hates mages because of Tevinter and that Anders hates Templars because of growing up in the Circle.
In short, DA2 used backgrounds and hot points to define their characters and their personalities... and this hardly ever changed. In addition, we never got to know anything about their background of where they came from outside of their personal tragic stories. And we never see their persona crack, showing us a side to them we don't usually see. We saw more of this kind of depth in DA:O and that's why I think many people like those characters more, but found the DA2 characters more one-note.