Negatives:
Huge agro pull if not careful. Medica I & II and Regen and quickly snap agro and kill you if you're not careful.
Mana Management - for pretty much every and all end game fights you'll need to constantly manage your Shrouds to make sure your timing them appropriatly to get you maximum mana regen and quickest recovery on CD

Positives:
Holy is seriously fun, love DPSing in WP's. Also very useful tool to stun mobs to allow healing of tank and DPS. Many times dps' are geting rocked that I pop swiftcast+holy then heal
Big heals. (Haven't played a SCH just from what i'm told) Sch's have a harder time topping off tanks that are dangerously low. Whm's we can do it pretty easily
Benediction - our oh shh button. My record is a 6800 HP Pld tank. 34 HP when Benediction hit, healed 100%. Love the button
Regen - also a negative because of agro but Looooooveeeeee the regen.

The way I related it is WHm = Holy Priest SCH=Disc Priests. I loved the Holy priest so I went WHm.