1) UCSF and mission Bay. A huge new campus that is bringing many more people to the area. Students and professionals alike will look for places to live near work.
2) The 3rd Ave light rail is now in. Once again, more things converting to residential where there were previously shipyards.
3) Whole foods will open (on the North side) this fall I believe. This store may serve as an anchor tenant to pull in other retailers. Long term impact or more retail means more people wanting to live on Potrero hill.
The biggest gotcha on the south side are the projects. This said, not all South side streets line up next to the projects (there are several south side locations in fact that are preferable to North side with respect to project proximity).
Bottom line, my layperson's opinion: if you can find a home you like on the South side and it' s not too close to the projects- it indeed may hold its value well (and if you like the home- who cares about the ups and downs in value ).
A simplistic scenario.... let's say a home is now worth $1 million on the North side... but you find the same home on the South side and negotiate a $850k purchase price (you have no negotiation power on the north side because there are too many Buyers willing to pay the price - but you have tons of neogtiation power on the south side because you have little competition). Later, the market is up 10% so the North sider sells for $1.1 million, but with Buyers swarming all over, they spread out to the South Side and think the same house for $1 million is a steal. In this scenario you get a 17.5% gain vs. a 10% gain.
In hot markets buyers spread out... the hotter the market, the further out they go. As the market slows, everything contracts again. The best properties in the best locations maintain their value the most ... (as your question implies), but you can "buy" your future appreciation in a slower market if you are a saavy buyer.
However, there are many plans for the area around the south side of Potrero Hill that should have a good influence on the area including:
1) obviously Mission Bay redevelopment which creeps all the way down to Maricopa St
2) The 3rd Street light rail
3) The new Espirit Park condo development on Minnesota and 20th in the Dogpatch (http://www.sfnewdevelopments.com/3466/esprit-park-update-2/)
4) The Pier 70 redevelopment efforts in the Central Waterfront (http://www.sfgov.org/site/port_page.asp?id=34925) which should help make better use of the 65 acres of waterfront property.
5) The Eastern Neighborhoods Community Planning efforts which could impact the Potrero Hill and Central Waterfront neighborhoods. It will impact the south side less so other than to keep it residential with the exception of a new Employment and Business Development District (EBDD) area near the projects on the south side. Additionally, there is a proposed "Life Sciences Zoning Overlay" just south of Mission Bay in the Central Waterfront which presumably would encourage the development of a "knowledge industry" in this area - startups, supporting facilities for Mission Bay life sciences research, etc. http://www.sfgov.org/site/planning_index.asp?id=25205
6) The increased redevelopment and commercial activity around the new "The Potrero" condo development including the new Whole Foods as well as Sega relocating their corporate headquarters across the street in the Culinary Institute building.
7) the constant discussion over either rebuilding or relocating the public housing projects on the very south side of Potrero Hill.
Hope this helps