Let's finish the analogy. Rose gardens and other community parks are usually community-funded; my local gardens are funded with taxes. There are not only paid moderators (police), but paid curators (gardeners and arborists) who deliberately build up and cultivate an appearance for the garden. Some of the more expensive gardens, like the local zoo, also have an entrance fee, because taxes alone would not fund the garden at its given size and occupancy.
There are communities like this; Something Awful is the first which comes to mind. These communities deliberately acknowledge that money is required to fund community spaces, and use the money to improve the space.
There are also extensions to the analogy. A local park has a bulletin board. Postings to this board are generally made by community consent; anything that any community member feels strongly enough about can be removed immediately. This is also how postings on telephone poles work. Sometimes a community will lock up their bulletin board after a wave of abusive listings. This is analogous to primitive message board moderation, as seen here on HN.
Are we here to advertise to each other, like on a bulletin board? Are we here to produce a great knowledge base, like in a garden? What should the shape of conversation be?