shield only blocks attacks in a cone infront of the player. and yes, it blocks spells (some spells are undefendable). passive defenses are effected by gear, tactics and your level compared to your targets level. im not sure if stats effect block rating, but i know initiative increases your chance to crit and reduces your chance to be crit. i have a feeling that it effects dodge, also, but i cant say for sure. you wont find any exact formulas at this stage, since mythic hasnt released any.

if youre playing any kind of tank you always want to go weapon/shield. do not use a 2 hander. the damage difference is minimal and the difference in survivability is massive.