Becoming a bard doesn't require a huge rigamarole. You used to have to multiclass something like 3 times.
Clerics have spontaneous healing and domain powers -- this is a significant ramp up in power.
Feats play a major role in the fighter class, making it both more complicated and more flexible.
Humans gain benefits instead of just lacking level limits.
All of the major spellcasting classes have spell level 0-9. Illusionists are now a specialized type of wizard.
Thief abilities are now all skill-based and no longer use a percentile roll.
Combat is optimized for miniatures. Technically, they aren't "required," but they simplify things unless you heavily house rule combat.
Monster challenges have a CR instead of just an HD match-up. They also can have class levels pretty easily. Templates are also a new concept. Monster listings always come with a verbal description.
There are more categories of stuff. Special abilities are (Su), (Sp), or (Ex). There are different types of damage reduction. Monsters have types that have a real impact on play. Weapons are simple, martial, or exotic.
No weapon speed, just light, one-handed, or two-handed categories.
Darkvision replaces infravision and/or ultravision.
Attacks of opportunity make combat more tactical -- or fudge it up, depending on your opinion. YMMV.
No electrum pieces.
Attributes have an impact every 2, e.g., Str 15 vs. Str 17. Even numbers change the modifier, while odd are used as prerequisities for things like feats and prestige classes. There is no exceptional strength (e.g., 18/00).
Prestige classes allow for classes with prerequisites. Not everybody can sign up to be an assassin. There is no shortage (heh) of prestige classes available.
Spell names have changes to be more consistent...for the most part. You will frequently see "greater" and "lesser" versions of spells, for example.
Characters automatically start at 1st level with maximum hp.
Saves are not only simplified to 3, but your attributes (Dex, Con, Wis) have a direct impact on every save in addition to your class progression.
Higher AC equates to better armor. Characters have base attack bonuses instead of THAC0.
Greyhawk is the default setting...which has little practical impact. Spell names and deities are the primary influences.
Metamagic allows you to pump up spells in a variety of ways, extending duration, range, damage, etc. Metamagic requires a feat (or rare magic items or spell components).
The rules for magic item creation are generally more consistent, and more data is provided for magic items in general -- aura, caster level, creation requirements, etc.
Multiclassing is much easier to do. There are no racial limits, and no "dual-classing." XP progression is regular across classes.
Sorcerers and bards are spontaneous casters, which is different from the 1e system where everybody memorized spells for the day.
For whatever reason, the sci-fi elements that were in products like Needle and the Expedition to the Barrier Peaks have failed to manifest in any 3e products, but WotC mostly relies on Dungeon and D20 publishers to produce adventures these days. The DMG does still have rules for laser rifles.
