Here's a list of important differences between the two editions, though I will completely agree with KRyan, D&D 3.5 is just so much better, inarguably so than its predecessor.
- Bards received more skill points and spells on their spell lists were changed.
- Clerics are allowed to spontaneously cast cure and inflict spells of the "mass" variety.
- Druid Animal companions advance as the druid levels up now, making the druid more playable, Druids were given access to far more spells than before.
- Fighters Feat list received an overhaul and many of the feat descriptions changed.
- Monk's Flurry of Blows was changed to have different advancement, and received different options for bonus feats. (It didn't help much in the long run, Multi-attribute-deficiency cripples this class)
- Paladin's can summon their mounts instead of them being ever-present. They can also smite evil more times per day.
- Rangers received a hit die increase, more skill points, Favored enemy and animal companion also received large revisions for the class, making it playable.
- Sorcerers were allowed to make small revisions to their spells known when they level up (but still have stunted casting when compared to wizards, picking up their spells a full level later)
- Wizard spell specialization was changed, and Familiars bonus stats were changed as well.
- Alchemy was rolled into Craft(Alchemy)
- Animal Empathy was removed from the game and instead Ranger and Druid now get the skill as a class ability.
- Innuendo which before was used to covertly pass messages along was rolled into the Bluff skill.
- Intuit Direction was rolled into the new Survival skill.
- Perform was rolled into its own skill, which works similar to the Craft and Profession skills.
- Read Lips was removed from the game and rolled into the Spot skill.
- Ride no longer is specific to different mount types.
- Pickpocket was rolled into the Sleight of Hand skill.
- Scry was removed, and now only relies on the spell Scry.
- Wilderness Lore was rolled into the Survival skill.
- Ambidexterity and Two-Weapon fighting were rolled into each other so that Two-Weapon Fighting provides the benefits of both feats.
- Expertise was renamed to Combat Expertise
- Weapon Finesse was redone, and now provides a benefit to every light weapon instead of requiring a new taking of the feat for each individual weapon that needed to be taken advantage of.
- Sunder was rolled into the Sunder special attack, and the Improved sunder feat was added to avoid Attacks of Opportunity while using Sunder.
- Shield Expert was rolled into the feat Improved Shield Bash.
- The feat Improved Critical was changed to not allow stacking with other Critical Range Enhancing effects.
The following spells were added to the game:
Acid Splash, Animate Plants, Arcane Sight, Arcane Sight (Greater), Baleful Polymorph, Bear's Endurance (Mass), Blight, Bull's Strength (Mass), Call Lightning Storm, Cat's Grace (Mass), Command Undead, Cure Critical Wounds (Mass), Cure Moderate Wounds (Mass), Cure Serious Wounds (Mass), Daze Monster, Deep Slumber, Dimensional Lock, Disrupting Weapon, Eagle's Splendor, Eagle's Splendor (Mass), Enlarge Person (Mass), False Life, Fox's Cunning, Fox's Cunning (Mass), Glibness, Heroism, Heroism (Greater), Hold Monster (Mass), Inflict Critical Wounds (Mass), Inflict Moderate Wounds (Mass), Inflict Serious Wounds (Mass), Longstrider, Moment of Prescience, Mordenkainen's Private Sanctum, Owl's Wisdom, Owl's Wisdom (Mass), Polar Ray, Prying Eyes (Greater), Ray of Exhaustion, Reduce Person (Mass), Shout (Greater), Summon Instrument, Symbol of Weakness, Sympathetic Vibration, Touch of Fatigue, Touch of Idiocy, Undeath to Death, Waves of Exhaustion, Waves of Fatigue.
The following spells were completely removed or changed:
Animal Friendship, Emotion (Fear), Emotion(Friendship) became Charm Person, Emotion (Hate), Mass Haste, Negative Energy Protection became Death Ward, Nystul's Undetectable Aura became Nystul's Magic Aura, Polymorph Other became Baleful Polymorph, Polymorph Self became Polymorph, Symbol (Discord), Symbol (Hopelessness).
The following spells had their names changed:
- Change Self → Disguise Self
- Charm Person or Animal → Charm Animal
- Circle of Doom → Inflict Light Wounds (Mass)
- Command Plants → Control Plants
- Emotion (Despair) → Crushing Despair
- Emotion (Hope) → Good Hope
- Endurance → Bear's Endurance
- Enlarge → Enlarge Person
- Greater Dispelling → Dispel Magic (Greater)
- Healing Circle → Cure light wounds (Mass)
- Improved Invisibility → Invisibility (Greater)
- Invisibility to Animals → Hide from Animals
- Invisibility to Undead → Hide from Undead
- Mass Charm → Charm Monster (Mass)
- Minor Globe of Invulnerability → Lesser Globe of Invulnerability
- Protection from Elements → Protection from Energy
- Rage → Emotion (Rage)
- Random Action → Confusion (Lesser)
- Reduce → Reduce Person
- Resist Elements → Resist Energy
- Symbol → Symbol of (Death, Fear, Insanity, Pain, Persuasion, Sleep, Stunning)
- Teleport Without Error → Teleport (Greater)
- Vanish → Teleport Object
On top of that, Many spells had their spell schools changed as well as some substantial changes to spell levels as well.
The spell effects of the following spells were changed:
Blade Barrier, Blindness/Deafness, Call Lightning, Endure Elements, Eyebite, Flame Arrow, Harm, Haste, Heal, Neutralize Poison, Otiluke's Freezing Sphere, Polymorph, Reduce Person, Righteous Might, Scrying, Scrying (Greater), and Wall of Force.
Along with those, many Domain spells were changed to either different domains or removed from domains entirely.
Damage Reduction Changes
Damage reduction was changed to allow many different types of things to bypass that damage reduction, Including Special materials such as Silver, Adamantine, and Cold Iron; Weapon Types, such as Slashing, Piercing, and Bludgeoning; Alignments, such as Good, Evil, Lawful, and Chaotic; Magic and Epic were also added to indicate physical damage reduction, and epic damage reduction. These factors were also combined to indicate monster weaknesses to certain aspects.
Equipment Changes (originally by Sean Mcmillian)
Equipment is now listed in small and medium-sized version for use by small and medium sized characters. The largest effect of this adjustment is the Shortsword/Small Longsword issue, which means that small characters will usually do one die-size less damage.
(The medium shortsword and small longsword both do 1d6 damage. In 3.0, only medium sized weapons were listed, and a gnome rogue could use the medium shortsword. In 3.5, our gnome needs to use a small longsword to get 1d6 damage, which requires martial weapon proficiency, or a small shortsword, doing only 1d4 damage.)
DMs should take this into account and make sure small characters can get small weapon rewards (or just house-rule it away.)
That's all of the changes that you'd likely be concerned about for just the basic version of the PHB from 3.0 to 3.5. The version change was indeed a greatly improved version of the original ruleset, and removed a lot of the cheese (Psionic's Handbook 3.0 Anyone?) as well as getting rid of a lot of useless skills and making a lot of classes very much playable compared to their 3.0 counterparts (Ranger and Druid spring to mind), Druid got a lot more spell access and animal companions are very nice in 3.5.
For those of you seeking a full version of all of the changes they can be found here on the wizards website.