Roughly Drafted claims to have knowledge of some of the possible new features coming in Snow Leopard. Unfortunately, the article appears to include some degree of speculation making it difficult to tease out what might be actual features.
One of the more interesting possibilities includes the introduction of full Resolution Independence. The groundwork for this feature was included in Leopard, but full implementation was not expected until a later release. The author speculates that the dramatic size reductions in the application sizes could be, in part, due to resolution independence. Besides file-size reductions by eliminating large bitmaps, Apple's research into resolution independence opens the door to ultra-high resolution displays. This could pave the way for Apple's previously rumored high resolution Cinema displays.
The benefit of resolution independence would be the ability for the operating system to scale its user interface smoothly to accommodate higher resolution displays. At present, most displays are around 100 dots-per-inch (dpi), meaning 100 pixels for every inch of screen display. If Apple were to introduce an ultra-high resolution display, it could have a dpi of 200 or 300. The same image viewed on these higher resolution screens would appear 2x to 3x smaller. Simply scaling up bitmapped images to fit the display would result in blocky/jagged images. If instead, they are described as vectors, as Apple is proposing, larger displays could smoothly scale the user interface to the size of the display.
The article also suggests that 3rd party support for the multi-touch trackpads, expanded data detectors, auto activation of fonts, and ZFS support (which is listed in features for Snow Leopard Server) are also coming.
Update Some have suggested that the reduction in Snow Leopard's filesize is simply related to languages and interface builder (nib) files, and that resolution independence does not play a role. Pipian demonstrates this effect.
