PvE
Armor Cap: 33K (50%)
Max Monster Armor / Penetration Needed: 18.2K (Tanks in organised groups may provide most of that, e.g. 14K to 16K)
Speed Cap: 200% (100% over normal non-sprint running speed)
Health / Mag / Stam: No cap, but generally not efficient to stack
Weapon / Spell Damage: No cap, tends to be more efficient to stack than mag / stam (better multipliers)
Crit %: 100% (very hard to reach without sacrifice, other than by using Acuity, thus effectively no cap)
Crit Damage Cap: 125% (75% above your default 50% bonus)
Block mitigation: Possibly 91% (based on my own experiments)

PvP
Armor Cap: The penetration of your attacker + 33K. Sky is the limit. In practical terms 45K as a troll tank, but not if you want to do damage.

Max Penetration Needed: As much as possible, but it's all a tradeoff. At some point (20K) you're probably better off worrying about crit or self-healing (more weapon / spell damage / crit % / crit damage). Really the only way most people get to 20K+ is with Balorgh, Corrosive Armor, or Onslaught.

Health: No cap. You can make do with 25K, less as a ganker. 30K is preferred as a Brawler.

Crit Damage Cap: 125% + the target's crit resistance. This is usually more than you can realistically achieve. Can't have enough on certain builds such as NB, due to guaranteed crit from stealth, or a Mechanical Acuity build. Not the be all / end all, but a viable way of building.

Crit Resistance: No cap that I'm aware of, but a compromise to stack other than with Rallying Cry.