Never played Dark Souls, wanna enlighten me?
Your stamina bar quickly recharges, the more weight you have geared on you the slower your recharge. At base human levels (not superhuman) you have enough to make about two to four swings depending on the weapon and whether it was a weak or strong attack, it takes very little time for your stamina to recharge, but as we all know even one second is an eternity in combat. Running drains stamina, so does attacking. If you are struck while blocking then you also take a stamina drain (And holding down block severely slows down stamina recovery). Parrying (something basically like the chambering we have now, timing a block where the opponent is briefly stunned and for a split second defense s set to zero) takes almost no stamina. If your stamina reaches zero or negative, then you are unable to swing or block until it reaches positive again. If an attack drops your stamina to zero, then extra damage will leak through as well as you being stunned (Punishing you for being passive, or not dodging something massive).
It basically makes players ideally do smart moves, punishes both spamming and turtling while rewards players who do constant flowing attacks interchanged with footwork or a few powerful well-timed attacks depending on your style. You need both reflexes as well as brains to actually not get murdered against an equal opponent, I love the stamina system as it keeps things dynamic and removes turtling/passive/spamming mentalities as well as heavily rewarding footwork and timing.
Granted, in both DS games this beautiful melee system encountered problems the instant a caster throws around sorceries/miracles (The first DS game spells were OP), but in Mount and Blade luckily magic does not exist =P