I find this discussion very interesting, and it's something I've been thinking about for quite some time. I do think that there is something special about Toronto's low-rise neighbourhoods (I live in one of them myself), but I also agree with others that there is room for growth and change in at least some of these neighbourhoods.
I also take issue with the accusation that gets thrown around (not specifically calling you out Tewder, it seems to be a common argument) that by desiring to see certain types of growth in Toronto, people are actively wishing to be like another city, at the expense of what makes Toronto what it is. Like I said, I agree that low-rise neighbourhoods are among Toronto's most beautiful areas and do contribute to a unique urban character, but that doesn't mean that we should be constricted by them. And yes, there are things that Paris, Barcelona, New York, etc. do well that would be nice to see in Toronto, and I think it is possible to learn those lessons without becoming "some second-rate version of somewhere else." It is important to remember that those cities became what they are over by going through their own periods of growth and change. New York today hardly resembles the city as it was in the late 19th century, when it was predominantly low and mid-rise (pre-high-rise boom). The difference is even more dramatic the further back you go. I'm sure that if you had asked a New Yorker in 1850 to describe the character of his/her city, it would be very different from the answer someone might give today, and thats ok. In Toronto, we shouldn't necessarily be stuck in a certain mould just because we hit our boom 100+ years later.
Then again, perhaps we should recognize the value in low-rise neighbourhoods that function successfully today, whether they be higher-income enclaves or lower-income subdivided houses. There is always room for improvement, but if it ain't broke (for the most part), don't fix it.
As others have said, we need to find a balance between preservation and progress. If Toronto is to grow sustainably, there are going to have to be some houses - probably even some beautiful old victorians - torn down. I think that our focus should lie in identifying that which deserves to be saved, whether individual buildings of architectural merit or entire neighbourhoods, and then restricting zoning in those places (or compiling comprehensive heritage lists) and loosening it in other places to allow for more natural growth. The key to this is to recognize that not every victorian neighbourhood in the old city is going to be (or, in my opinion, should be) saved.
Also, as a note, and as others have pointed out, there are certainly ways to increase density incrementally and while keeping a low-rise built form. Simply allowing smaller multi-unit buildings to be built on some of our smaller streets may be a good way to do this.
