I think the Byward Market has always been the commercial centre of Ottawa. Downtown has always had a lot of offices and government. From the few historical photos I've seen, I believe much of the area covered by apartments and offices was once a haphazard array of plain wooden houses, much like Hintonburg. Bank, Elgin and Sparks appear to be the main historic downtown commercial streets, while Wellington is all government. The other side of the canal (the market) seems to have functioned much more as a traditional "downtown" over the years.
Darkstar: I think the area around Somerset and Bank is the gay village. West on Somerset is Chinatown (I was a bit surprised Ottawa has a Chinatown). There is also something called Little Italy yet further west, but it doesn't amount to much as far as I can see.