That's the way it always is with any recast, it takes a while for everybody adjust: it takes the performers a while to adjust to get used to the feel of the character, but find a little bit of the character in themselves to make the characters their own - as I've said before, nobody's done that quite like Marty did with Telly and Snuffy, he quite fantastically breathed new life into two previously depressive, Eeyore-ish characters; and it takes us a while to adjust to the new voices and such, but after time passes, you do get used to it and accept it - I remember how long it took me to adjust to Eric doing Frank's characters and David doing Cookie, but I don't even think about it anymore, and haven't for years now.
Zoe had a unique voice to begin with, mainly because Fran had a unique voice, and all things considered, Zoe was pretty much tailor-made for Fran (so much so they considered naming the character Frannie in honor of her, but she didn't want that). Jen's just starting out with Zoe, so it's hard to judge the performance and voice for now . . . I mean, yes, this is so far one of the more obvious differences we've noticed yet, but surely Jen can make Zoe her own with time.
As for Gladys, I think that can be forgiven - was ever really that major a character to begin with? Even when Richard did her?