• Hit points. In 1st edition you rolled 1d4 hit points as a first level wizard. If you had a constitution of 15, you could get a +1 bonus, or with a 16, a +2 bonus. That was it. The maximum possible hit points for a first level magic-user were 6. In 3.5 you started with maximum hit points for your hit die at 1st level. Additionally, constitution bonuses began at 12 rather than at 15, meaning that a lower score could result in a higher bonus than in previous editions. Finally, and most damning, was the Toughness feat. This innocent feat, considered underpowered by some, gives you +3 hit points at 1st level. Given your low hit points at 1st level, it was a good idea to spend your 1st level feat on it, and thus gain survivability. An average wizard in 3.5 could be expected to start with about 8 hit points. The theoretical maximum would be about 12, which means that a wizard could start with as many hit points as a fighter. The math at higher levels as well shows that 3.5 wizards have many more hit points than their AD&D counterparts.
• Bonus spells. In 3.5, you gain bonus spells for high intelligence as a wizard, just like clerics did in AD&D. So while the AD&D wizard starts with exactly one spell per day, his counterpart in 3.5 starts with 3 0-level spells and usually 2 1st level spells. It should also be noted that 3.5 0-level spells (cantrips) are significantly more powerful than the cantrips shown in the AD&D Unearthed Arcana. This trend of more spells per day for a 3.5 wizard continues into the higher levels.
• Spells Known. A limiting factor on AD&D wizards was a maximum number of spells per level known, and the necessity of rolling to determine if you understand a given spell. No AD&D wizard was guaranteed to possess every spell he wanted to. 3.5 wizards on the other hand, are free to choose which spells they want, ensuring that they will always possess the most powerful spells in the game. In addition, they have no maximum spells known, meaning that they could theoretically possess every spell in the game.
• Armor. In AD&D you could not wear armor as a magic-user, with the exception of elven fighter/magic-users, who are a different topic altogether. Additionally, there was no easy way of getting better armor for yourself. Your best low-level defensive spell, shield, lasted but 5 minutes/level. Additionally, the beloved displacement spell of 3.5 wizards was not in existence. Instead, AD&D wizards had to use the dangerous and unpredictable blink spell. In 3.5, armor had a “spell failure chance.” This was a chance that your spell would fail due to the armor interfering with the spell’s somatic components. A variety of methods (using mithril armor, taking certain feats, “dipping” classes, etc.) could easily nullify the spell failure chance, allowing for wizards to wear armor. Finally, even without armor, it is entirely possible for a 3.5 wizard to get an Armor Class of 42 by 20th level, or an AC of -32 in AD&D terms. The more a wizard can defend himself, the less he needs a fighter to defend him.
• Magic Items. In AD&D, one had to be fairly high in level to create the majority of magic-items, 11th level to be precise. Potions and scrolls could be made sooner, but the creation process for all magic items was exceedingly onerous and difficult. Additionally, in AD&D it was impossible to purchase magic items. In 3.5, potions could be brewed by 3rd level, wondrous items could be made by 3rd level, and magic swords and armor could be made by 5th level. More insidiously, it was permissible to purchase magic items in large cities. This means that any magic item you wanted could be either crafted or bought. The DM was no longer in charge of distributing magic items. Although this might sound good in theory, it means that any magic item you want can be yours. Since players always take what is best for them, it means that every D&D character will have the “good” items. This helps wizards inordinately, for now they can guarantee themselves better protection and better offensive capability.
• Spellcasting. In AD&D, spells took a set amount of time to cast, and taking even a single point of damage during the casting of the spell would disrupt and ruin your spell. It was imperative that fighters guard the magic-user, for if the party mage got hit, there would be no spell from him that round. In many cases, this could mean the difference between victory and defeat. In 3.5, you could make a concentration check if struck while casting, giving you a chance of not failing your spell.
• Magic Resistance. In AD&D, magic resistance worked on the principle that for every level below 11th, a magic-user has a -5% penalty on his magic resistance roll, and for every level above 11th, a +5%. If he fails, the spell does not work on the creature. 3.5’s spell resistance works on the basis that the wizard rolls a d20 and adds his level. If he beats the monster’s spell resistance number, his spell works. These two systems are very similar. The difference is that a 3.5 wizard can pick up the feats Spell Penetration and Greater Spell Penetration to give himself a combined +4 on the roll. Furthermore, he can use the Assay Spell Resistance spell from the Spell Compendium to give himself a +10 on the roll. Again, in 3.5 there are always ways to work around your limitations as a wizard.
• Saving Throws. In AD&D, saving throws got better as you gained levels. Difficulty remained static. Your spells as a magic-user were always the same level of power, but monsters simply became more resistant to them. This resulted in spells dealing half damage or being negated routinely by higher powered monsters. In 3.5, the difficulty of resisting your spells scales alongside the monster’s save bonuses. Your intelligence modifier and the level of the spell that you are casting, plus feat modifiers, all apply to the difficulty class of your spell. With a bit of work, you can make even powerful monsters routinely fail their saves versus your spells.