Anyone have any good summaries of how 3.5e rules were different than 5e rules?
Biggies off the top of my head.
1 concentration for magic is different. Most spells do not require concentration so you can stack buffs and such. 3e concentration is a skill check you make when hit, or to cast without provoking attacks of opportunity.
2 each slot is its own specific prepared spell for most casters (prepared casters are wizards, clerics, druids), only sorcerers and bards with spells known do slots straight with flexibility like in 5e. If a wizard wants two 1st level magic missile spells they have to prepare two of their first level slots as magic missiles.
3 many spells go up by caster level. A fifth level caster's 3rd level slot fireball does 5d6. A 10th level caster's 3rd level slot fireball does 10d6. Maxing out caster level is a big deal for magic power.
4 Spell saves are based on stat bonus + spell level, so higher level spells are harder to save against.
5 No bounded accuracy, stats are not limited to 20 and numbers keep going up.
6 only three saves, fortitude, reflex, and will with bonus based on class, class level, and stat.
7 Attack bonus varies by class. Warrior types go up 1/1 with levels, others go up 3/4 or 1/2.
8 Extra attacks are based on class base attack bonus unmodified by anything else. You get an extra attack at +6, +11, +16 but each new one is at -5 worse than the one before. Maxing out base attack bonus is a big deal for weapon attacks.
9 To use extra attacks you have to do a full attack, which means you can only move 5 feet. If you move more than 5 feet you are limited to one attack.
10 Everyone gets feats at level 1 and every three levels thereafter. Feats are generally less powerful than in 5e but you get more of them. Monsters have them too.
11 death is different. No death saves, at 0 you are unconscious, at negatives you lose one hp a round and die at -10. You have to heal from where you are at negatives, not from 0.
12 No hit dice for recovering hp on a short rest, only 1/hp per day of rest, magical healing is the norm.
13 a lot more effects bump up or knock down your stats with a lot of recalculation of numbers.
14 skills are done out by points per level so you can customize the exact modifiers and you get more points each level (skills are capped by character level). You get skills from class and int bonus each level, but many classes only get two skills so if they max them out they are about half as skilled as they would be in 5e.
15 A lot of actions provoke attacks of opportunity (leaving any threatened square, casting magic in melee range, standing up from prone, others)
16 a lot of things are immune to sneak attacks.
17 In addition to AC there is touch AC which is AC based just on dodging type modifiers. This is different from reflex saves.
18 No backgrounds or inspiration.
19 Alignment has a number of mechanical impacts for interactions with specific spells, magic items, monsters, and some class stuff.
20 Some classes have built in roleplay stuff like the paladin code and losing powers for violating their code, this is generally more constraining RAW than 5e.
21 Attunement for magic items is not a thing, some magic items take up specific body type slots though (only one set of gloves, one belt, one headband, one cloak, one magic ring per hand, etc.), and the baseline math includes assuming advancingly more powerful items of certain types (mostly attack and defense and stat buffing) as you level.