Items that formerly granted a "resistance" such as rings or object properties on armor, grant full immunity. Races and roles that have starting "resistances" get immunities in FIQhack as well, but level-up resistances, and any other sources of a resistance only halve the damage. Items that grant a resistance or an immunity will be labeled appropriately if you are confused, and the Oracle offers enlightenment as a service for gold!
On the plus side, powerful spellcasting monsters such as liches no longer have dangerous melee-range spells such as destroy armor and drain strength. However, they are perfectly capable of casting problematic spells like slow monster and fireball/cone of cold. Elbereth can give you far less safety, as casters and monsters with ranged attacks or wands will be happy to stay away and bombard you. Prepare accordingly.
Dragons are now much more dangerous, since your resistances will only halve the damage. Watch out for yellow dragons!
Weapons and Armor
The cloak of displacement has been buffed to give MC3, and the cloak of magic resistance now gives MC2.
All DSM except for silver and gray have been buffed. If you can get reflection and magic resistance elsewhere, consider blue and orange DSM, which grant speed and free action respectively, in additional to providing valuable immunities.
Easily the most important change in FIQhack is the introduction of object properties - while they're not as insane as GruntHack's items, for many characters, object properties can be a valuable source of immunities. Most magical armor and jewelry have relatively mundane properties, but they can be easily identified using a source of enlightenment. The advantages of possessing numerous immunities can make Fort Ludios an appealing location for players looking to fill out their kits.
For players early in the game or those who choose not to use artifact weapons, weapons with properties can also be very useful. Be careful of wielding magical weapons, even uncursed ones, as the mercy property autocurses and converts your damage to healing. Thirsty, fire, cold, and shock weapons deal an extra 1d6 to non-resistant targets and can be easily identified by throwing the weapon up while wearing a hard helmet.
Wands / Attack Spells / Dragonbreath
The above can now ignore reflection, so you'll want not just reflection, but also resistances or immunities to all elemental damage types, if possible. Keep in mind that crowning has been buffed and provides intrinsic acid resistance. At least either fire or cold immunity is recommended.
Having expert skill in wand and a blessed wand equals "master" level skill effects. For many wands, expert skill doesn't matter, but for many wands it does; here's a selection of some of the best effects:
<FIQ> if you want to abuse special levels with master wands,use locking <FIQ> it replaces doors/doorways with a secret door, acting as a wall until you uncover it <FIQ> other nice things you can do with master wand <FIQ> light up entire levels <FIQ> detect traps with SDD wand <FIQ> cancel '''everything''', including Rodney, or riders <FIQ> use polymorph offensively and have it actually be useful
Expert divination allows you to detect fooproofing status with the Identify spell.