Your avatar makes me wish Leif is back lol. He was one of my fav new special characters. Hopefully he will be confirmed soon.
I drew him recently at that made me think about the guy. I didn't care much about the New Leaf characters because they just didn't have many dialogues until the Welcome Amiibo trucks came around...
They decided to streamline everything shop-related so after a while it felt like talking to vending machines whenever you went to the shop area.
Blathers specially. Maybe it's because of the contrast with earlier games, but I could swear he only had lines for "thanks for your donation, this reminds me of something i won't tell you about" and "we already have that, sorry".
That's my main worry with New Horizons, basically. I want to take my time talking with everyone. Lots and lots of dialogues and situations, maybe have some errands you can do for the non-vilager characters, more backstories for them...
If it's the same as New Leaf, I won't really care that much about the new crafting mechanics since that'd mean the game is only geared towards decorating the town.
How were the special characters in New Leaf? They were my favourite part in WW and CF, hoping we get some great ones this time again.
^^Since this seems to reply to you as well...
At first they were great, though. It's just that most of their dialogues and scenes take place when you enter the shop for the first time, so you kind of expect that they keep having new things to say, but nope.
As for character designs I found them fun, to be honest. The Re-Tail couple was easily the best, but they also expanded on Kapp'N and his family. I really hope this new game expands on Leif, the flower shop guy and Luna, the Dream Suite host, since those were new ones that suffered the most from just being there for the shop.
Eventually you could unlock the Coffee Shop and the Welcome Amiibo trucks and squeeze some dialogues out of them, but I don't remember ever encountering them in those places, as it was random who would appear out of all special characters.