Honestly I think the new characters are wonderful, and they all seem new to me. Mako, while looking like Zuko, is probably going to have more of a strong and silent personality. Also, if I remember correctly his voice actor, David Faustino, once described him as a confident ladies man, so no, he's NOTHING like Zuko =P. Bolin, considering that in a drawing that was shown at the panel he was reading a book on "how to get a girl" or something like that, he will probably be really, really awkward. Also, just from the three words he said at the end of the trailer he will definitely be the Sokka of the show.
As for Tenzin, I'm thinking that he would be like Iroh, but I'm doubting it considering that Mike and Bryan are too creative to recycle characters (not that I would mind it if they did. I love me some Iroh

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And honestly I don't think that this is too modern. The Fire Nation was already in the middle of an industrial revolution in the original show. I mean for god's sake 70 years ago they had tanks, airships, coal burning warships, submarines, the basic idea of tram systems, etc. Quite frankly, this progression makes complete sense to me. After all, we went from horse and carriage to planes (not commercially, of course), trains, and automobiles in less than 70 years. Also, the skyscrapers make total sense to me because of Earthbending.