I like rooms that look like real people's rooms, with mismatched objects and weird collections, and not like furniture showcases for one set. Rooms that use more than one series are my favorites, especially if there are also a lot of non-series items in them.
In terms of series, I like the Alpine series and the Green series when it's remade into dark green. I also like the polka dot series, and I kinda like sleek and astro stuff as well.
Anyway, to answer your original question - I don't like houses that are just all a specific set. I have a full set of StreetPass houses, so it gets boring to see rooms that are all ice set or all balloon set or things like that. I really prefer rooms that people put more thought into. Maybe it has a specific theme (looks like a library or a restaurant or other things like that) or maybe it looks like a real person's house (a kitchen room, a bathroom, a bedroom, etc) or maybe it's trying to be really unique (mazes or other custom things).
I'd have to agree with the sweet set and the bug set. Though, I usually like making rooms based on my life (as in, your career or your major might be an interesting set up).