Let's see:
Imagine a ruleset where a lot of the tables were replaced by standardized formulas. No 6 tables for ability scores with different benefits, some of which only apply if you're race X or class Y. No big tables for saving throws.
Imagine a ruleset that is more tools than rules - instead of saying "you're not allowed to do X", it lets you do it, albeit with a disadvantage attached to it. Or it just goes ahead and lets you do it.
Every race can have every class (restrictions like that belong to settings, not rules, anyway), and few classes have alignment restrictions. No ability requirements, either (well, you need a certain Int, Wis or Cha if you want to cast spells, but you could be a rogue with Dex 8 or a Fighter with Str 3)
You can combine all classes with each other, and you can do so on a level-by-level basis. No different systems for multi-/dual-classing based on race, either. The basic bonuses on saves and attacks will stack, not overlap.
The system has been standardized so higher rolls and bonuses are always better: AC is better, the higher it gets. You try to get over that with attack rolls (the higher, the better). Saving throws get higher and they need to beat save DCs.
The basic action is almost always a d20 plus bonuses that needs to beat the target number: Attacks, Saves, Skills, Ability Checks. The bonuses are usually something derived from class and level and something derived from your abilities. That's right: The agile rogue (that's the new thief, without the burglary prejudice attached) is more likely to evade that fireball than the clumsy one.
A character's abilities affect how hard to save against his powers, too. The smarter the wizard, the harder to get out of his fireballs way.
Characters are a lot more customizable, too: Instead of Non-Weapon Proficiencies (and Rogue Skills), there's Skills and Feats now. Skills are things you can get better at over time, like hiding, crafting things, bluffing, knowing about hostory or religion. Feats are one-time abilities that give you new abilities, let you focus in all kinds of abilities, or lessen penalties for difficult actions (like fighting with two weapons at once). This also includes the whole weapon proficiencies thing (note that everyone can get proficient in any weapon, by spending a feat on it. All classes get their standard loadout of weapon proficiencies from the start).
You get one feat every 3 levels (and another one at level 1; some classes get bonus feats, too.) and several skill points per level (2 for fighters and such, and 8 for rogues. Everyone gets his int bonus on top of that every level, too).
One of the greatest things about D&D 3.5 is not that it's so easy and streamlined, but because everyone may use it and makes his own stuff based on it. And they don't owe no one a dime. They can even sell it. In fact, there's a lot of really great 3rd-party stuff out there.
The rules are online, too
http://www.d20srd.org/ The System Reference Document doesn't contain everything (character creation and advancement as well as some iconic D&D monsters like Mind Flayers are found only in the rulebooks), but the rest is in if you want to take a peak (the races are there, the classes are there, the skills, feats, spells, weapons and other equipment, the monsters, from the core rules and some other rulebooks, like psionics).