If you just want smaller and lighter, but doesn't need to be 'pocketable', and need reasonably fast focus with possibility of having some continuous/tracking focus ability, the current mirrorless cameras widely reviewed to have that ability are the Sony A6000, Olympus EM-1, Panasonic GH4, and Samsung NX1, with the Fuji XT-1 maybe a notch behind those. Most other mirrorless cameras can single focus very quickly, but won't have as much ease with following focus on a moving subject. All of these have on-sensor PDAF systems which have pretty solid tracking ability when set right and used right, except the GH4, which uses a specialized CDAF system that analyzes defocus through depth and lens analysis. Each system seems to work pretty well in sport and animal tracking tests compared to DSLRs - equivalent or better than most entry level and in some cases near mid-level DSLR performance...though not with as much low light sensitivity. The other system generally fast enough for action and tracking focus would be the Nikon Series 1 cameras - though they can be quirky with limited controls and drop off quickly in low light, plus with smaller sensors they don't compete with the M4:3 or APS-C sensor cameras at higher ISO levels or dynamic range.
