It's no secret that Park Merced, the sorta-suburban mega-housing complex in the southwest corner of the city, wants to expand. New mid-rise towers would house some 7,000 apartments, with space for maybe 12,000 new residents -- which is fine if you like the idea of more rental housing in the city (although much of it not affordable). But it also means a huge amount of new traffic in the area, particularly on 19th Avenue , which is pretty crowded as it is.
Now, the developer and the city talk about adding new transit to the area -- an underground Muni rail station at Park Merced, more buses, all that good stuff. Sup. Sean Elsebernd, who represents the district, is (properly) demanding it.
But here's the hitch: Never once in the history of this city has a major new development paid enough fees or brought enough money into the city to pay for the infrastructure required to serve it. And that's going to get even worse if the mayor gets his way and defers development fees. 
The cost of the level of transit necessary to serve the new residents of Park Merced, along with the expanded number of students at San Francisco State, and the expansion of the Stonestown shopping center, is gong to be massive. Park Merced may pay to build a new station -- but the developer won't pay for the cost of buying new buses and trains, hiring operators, and paying them. The increased property tax revenue from the project won't cover that, either -- particularly since it also has to cover water and sewer expansion, police and fire expansion, new schools and parks, and all the other expensive things that 7,000 new residents will want.
I don't think the city's even come close to figuring out the total bill for all the infrastructure improvements this project will require. Let's add that up first -- before the city issues any permits -- and present the developer with the bill. Then we can decide if this project is a good idea.