1. Enclosure. Briefly, space can be assigned in a passive or active manner. Structures, such as houses, apartments, offices, retail, and so forth, are active insofar as they respond to their context; the space between, whether it is used as streets, sidewalks, parks, gardens, courts, quads, or whatever, is, by contrast, passive. Passive space isn't built for a particular activity, and as a result, all sorts of activity may happen in it. Enclosure is the way surrounding active space responds to the passive space so as to certain kinds of activity in the passive space and not others--that is, it activates it. When this activization process fails, passive space goes unused.
But to activate a passive space, there must be a critical mass of activity surrounding it. Even large parks are activated in such a manner: witness Central Park, Fairmount Park, Hyde Park, or Balboa Park. Enclosure is thus an act of definition above all else: if the opposite process is attempted, the interspersion of active uses in parkland, there is not enough of a concentration of active uses to effectively define and enclose a space: it cannot be activated, and hence becomes a barrier, both physically and mentally.
I have now dedicated two posts to what is wrong with the sweeping-parkland vision (it becomes a barrier vision), and why the Radiant City and Landscape Urbanism are paths to ruin.
2. Streets. Basically, there are two types of streets: complete streets and naked streets. Complete streets have dedicated lanes for each kind of transportation flow along it: traffic, pedestrians, bicyclists, public transit. Naked streets, also known as shared space, have no dedicated lanes, or signage. Rather, they are like pedestrian malls with traffic; driving and parking lanes, for example, are demarcated by changes in road texture. These two synergize with one another. Complete streets, with modal dedication, make sense for faster arterials; naked streets, with modal interspersion, work better in neighborhood contexts. Complete streets narrower than 50 feet are unlikely to work, as are naked streets wider than 50 feet. Any street less than 20 feet wide, however, needs to be naked.
Here is some more information.