I was building a city which was mostly R$$ when it started to have some pretty bad abandonment issues. The population at the time was around 8,000-9,000 so not very big. Commercial and industrial demand was in the toilet because the neighbor cities were both full of it. Also, demand for residential was through the roof. So it puzzled me that they couldn't find jobs. Then i came across a solution. I checked my neighbor connections and they appeared to be solid. There were multiple rail, monorail, highway, and avenue connections (I never use road connections, they give me traffic headaches). I looked at my avenues and realized that the sims couldn't get to thier jobs because i hadn't built a place for the sims to reverse direction. So at each avenue connection i built a road connecting the two sides so that the sims could turn around. Voila, no more abandonment. This seems like a very obvious thing but i never read anything on any forum regarding it. Obviously it wont solve any massive problems, but for those small localized issues of abandonment, this might be your solution.