Bathurst and Don River work well for simplicity's sake. Bathurst to Don I guess is "central" (I don't think everything in it is "downtown"). The "feeling" generally drops off around Spadina and Jarvis IMO. Harbord Village for instance basically feels like a west end neighborhood to me. Cabbagetown is kind of its own thing. The transition from downtown to neighborhoods is smoother going west than going east, as the east-central area (i.e. Jarvis to the Don) was much more impacted by urban renewal than the west-central area (i.e. University to Bathurst). In addition, east-central Toronto developed very early and east of the Don development was delayed, while westward development was more contiguous.
It's when you go up north it gets confusing. I tend to stick to North Toronto for what was once the Town of North Toronto, since it does seem to be a clearly definable area that's larger than a neighborhood and more of a section. It's the Bloor to St. Clair area - the Annex, Casa Loma, Deer Park and Rosedale - I'm not sure what to call.