Death Spells 

Instant death spells are far more dangerous in a world without raise dead. In an S&S world they do not kill instantly, instead they take you to -1 HP with all the consequences that entails.

Sample Death Spells- Slay Living, Finger of Death, Power Word Kill.

Massive Damage 

If you want more realism in your combats then taking damage more than your Constitution should force you to make a massive damage saving throw DC is equal to damage. If saving throw is failed you are automatically at -1 HP & dying.

Hit Points Equal to Constitution 

For hyper-realistic campaigns characters get HP equal Constitution. For every size level above large double a monster’s HP. For every size level below small half a monster’s HP.

Monsters without any Constitution should either use HP as given in the monster manual or get 3d6 HP modified by size and whatever other modifiers the DM wants to apply whichever the DM deems more appropriate. 

If you use this system rogues roll d6 per sneak attack damage dice. On 1-2 they do no additional damage. 3-4 +1 additional HP of damage. 5-6 +2 additional HP. Basically d3-1 per dice of sneak attack.

If you choose to use this system damage spells should never improve with caster level. They always do the basic damage given in the spell description. Any change that affects Constitution automatically affects HP. 

Variant. A slightly less fierce variant of this which still allows some HP progression is to allow characters to gain HP equal to their Basic Attack Bonus. Fighters and barbarians thus gain an extra +1 HP at first level and an extra HP per level thereafter and so on. If you want to be really generous as a DM you can give double the Basic Attack Bonus as extra HP.

En Masse Attacks 

Attackers get +1 to hit for every additional attacker after the first. This gives swarms of low level grunts some hope of hitting against high levels. It seems fairly realistic as well. 

Scary Monsters 

In S&S campaigns pretty much all monsters are unusual and scary. On first encountering them, characters must deal with the fear affect. This should be handled according to the rules for frightful presence (pp309) of CRIII.

Contest of Wills 

Sometimes the rules call for a contest of wills. This is merely a competitive die roll pitting both parties Will saves against each other. Highest roll wins.