Just to add to this...and many thanks to
The Steel Warriors as well as the pally and ranger boards for providing all of this info I'm posting below.
Shielding: Cap is 35 - Shielding appears to reduce a percent of a mob's DB(Damage Base). What that translates to is it being near worthless on experience mobs, but ideal for named mobs. A lot of people have the misunderstanding that shielding reduces a max hit, or even a % of whatever you got hit for at the time. Say you're fighting a mob who's minimum hit is 200 and max hit is 1000 (something in RS or Nest maybe). Having max shielding reduces the mob's hit about 55 points. So the mob will now hit you for anywhere from 145 to 945, 945 clearly not showing a 35% reduction in the max hit. Hopefully I calculated that right, but long story short - even maxed shielding won't benefit a BST much in the roles we usually perform. It will always help, because every mob has a DB, but you won't see a large reduction in their hits like you would if you tanked a named with a 2-3k DB.
Avoidance: Cap is 100 - Avoidance is believed to be calculated in increments of 10. So 10 avoidance = 1% reduction in a mob's hit rate, 50 = 5%, 100 =10%. Avoidance also reflects in your displayed AC, but doesn't necessarily grant you better mitigation. This falls into the realm of AC softcaps, displayed AC, worn AC, etc....which we may never fully know. There has been some debate as to the return past 50 or 60 avoidance, but the parses done by the enchanters were entirely too small to be accurate.
Combat Effects: Cap unknown? I've seen people suggest 100 - CE increases your procs per minute or PPM. The effects of CE were apparently reduced when Weapon Affinity was added as an AA. I can't find specifics on how it's calculated, but the two don't stack, but rather work together off your base PPM...for example a proc that has 2 PPM with WA5 should have 3 PPM. Having 50 CE doesn't give you 50% of that 3PPM, but closer to 50% of the original 2PPM. So you should end up with about 4PPM with WA5 and 50 CE on a proc that's base is 2PPM. I'm not sure if that's exact, again just going off my understanding. Also it's been shown that you can only proc once per swing, once per round per proc, a weapon's innate proc is checked before an aug which is checked before a buff proc, and you can't proc on a riposte. So essentially with our dual wield skill and low double attack from AAs, a BST can only do a total of four procs per round of combat. The odds of that being an issue are likely rare, but something to consider when asking for proc buffs or adding 2 proc augs to a weapon with an innate proc.
Accuracy: Believed to be capped at 150 - as said above, increases your chance to hit. Don't think I've seen an actual percent attached to this effect yet.
I believe they've shown Strikethrough, Spellshield, DOT Shielding, and Stun Resist to all cap at 35. Strikethrough already explained above. Spellshield and DOT shield reduce the damage done by a DD or DOT, including mana-drains I've been told. Stun resist is just what the name implies.
Cleave and Ferocity both going up to level 5. Cleave explained above, Ferocity being an increase in the chance to double attack. Along the line of effects that don't accumulate but only take the highest number, you have Improved Dodge and Improved Block both capping at 3. If you see something like our PoTime BP with Block 8%, that actually adds 8% to your skill level, increasing the number as it displays in your skill window. A Block % effect and Improved Block do stack, however, as one is an increase to your skill and one is an actual effect.
Guard is a melee proc that essentially casts a rune on you. You could look at it as another way of lifetapping or healing, only the rune is there to reduce or remove a certain amount of incoming damage. In the case of the top Guard proc, it will reduce incoming hits 5% until it reaches 200 total dmg mitigated, then it fades. In my opinion, it's probably one of the more worthless procs.
If anyone knows any of the above to be incorrect, please let me know. I'd rather be completely wrong and be shown the correct info than to think I'm right when I'm not.

Again, the Steel Warrior forums have tons of good solid info on these things if you have time to search there.