Yes, block value enchant. I havn't seen a block rating enchant yet.
Shield spikes are retroactive threat, while shield slam is active threat.
I think shield spikes can crit, but I'm unsure about how mob armor/resistance affects it's damage. I feel safe in assuming it's similar to 'instant bleed' effects. Such as the effect on despair, when it procs.
If so, it's unmitigated but avoidable, and can crit.
But, warriors are more reliant on consistent or active TPS for threat, as all of their abilities are reliant on actively participating. Which is unlike tankadins, who can rely on somewhat passive threat building (blocking = damage dealt; so getting hit causes threat).
I did try a shield spike when I was in the low 40's and had one tell I hit 60, and started doing MC/ZG. And honestly, it was, good enough and not at the same time.
Shield block value is threat AND mitigation. Shield block value is the second most important derived stat for tanking after time to live calculations. Shield blocks that occur during fights are tertiary in regards to the hit table, and what I mean by this is such;
If you have 490+ defense skill, 15% Dodge, 15% Parry, 9% miss, and 20% block. Your hit table will look something like this;
On a roll of a hundred sided dice;
1-15 = you dodged
16-30 = you parried
31-39 = they missed you
40-59 = you blocked
60-75 = you got crushed
76-100 = you got hit
Now, when you increase your block % somehow, say with the shield block skill. Dodge, parry and miss take precedent over block %'s. So +75% to block % table would look something like this;
1-15 = you dodged
16-30 = you parried
31-39 = they missed you
40-100 = you blocked
That's only 41% extra block chance. And it gets worse all the time as you increase in avoidance.
Summary;
-For tanking regular instances, 5-mans, leveling instances, shield spikes are a decent way to go in regards to threat. For tanking higher end content, stamina or shield block value are better returns for tanking.