These are the main things the D300s has over and above that of the D300.
1.D300s shoots HD movies at 720p resolution, 24 FPS with stereo audio. Maximum length is 5 minutes for 720p and 20 mins for lower video resolutions.
2.D300s is slightly faster than the D300, shooting 7 FPS in Ch mode (Nikon D300 is 6 FPS). With MB-D10 battery pack, it will shoot 8 FPS.
3.A new release mode Q (quiet shutter-release) is added to the dial right after Ch (continuous high speed).
4.Dual card slots the Nikon D300s features dual card slots to work with both CompactFlash and SD (SDHC-compliant) cards. Either card can be used as the primary card. Secondary card can be used for overflow or backup storage, or for separate storage of NEF (RAW) and JPEG images and images can be copied between cards.
5.Active D-Lighting now has Auto and Extra High added. Auto is something expected, as both D700 and D90 have this mode. The Extra High is something new though.
6.Nikon D300s is slightly heavier than the D300, adding 15 more grams of weight, weighing total 840g total.
7.Nikon D300s has a dedicated Lv (LiveView) and Info buttons on the back of the camera.
8.Nikon D300s has a virtual horizon now (D300 did not).