In my opinion the best descents on the Long Mynd in no particular order are:-
Round hill. Very long and fast double track descent which also features a few bits of single track. Can be prone to walkers though so try and do this early in the morning. No 9 on the map below.
Duckley nap and the batch. Very fast grassy descent leading into a sweet bit of singletrack. 11, 12 & 14 on the map.
Minton batch. Flowy singletrack with a few little technical features and (sometimes) angry sheep dogs chasing you at the bottom :-). 7 on the map.
In my opinion the best climbs up are up 1, 10 (Carding Mill Valley) and the tarmac road up from Church Stretton (the Burway).
Have fun!
http://www.plushhillcycles.co.uk/trails/105-long-mynd-mtb-map.html