One good rule of thumb is that a Fighter of double your level, with no feats, should be at least an even threat.
That is, you are not completely immune to him, and you can't just blast him in one round.
This breaks down at higher levels with spellcasters who have no-save spells. But it's a good guideline up to 10th level or so.
This isn't just a 3E problem. In 2E with the "Complete X Handbook" stuff you could make an Elf Fighter (Archer Kit) who could machine-gun everything and fought as if he were a basic 7th level Fighter at chargen. You could also play a Fighter/Thief, putting all your weapon proficiencies into Unarmed Specialization, and be from Teziir. You'd get the attacks per round of a 7th level specialized Fighter at chargen, and deal roughly enough damage to kill a 4HD Ogre automatically every round (no real chance to miss). An unlucky DM would let you play a 1st Edition Monk (or, to shudder, a Fighter/Monk) from Teziir.
This ignores the potential for anyone to blow a ton of Nonweapon proficiencies to get martial art stuff from the Ninja's Handbook, with the potential for +5 AC, +1 attack / round, and damage one die type higher.
And 1E was pretty crazy in places too. The Oriental Adventures characters were all better than the standard ones, in fact about thrice as powerful. Using stuff out of the Unearthed Arcana your character could easily have incredible stats and high starting HP. And both books features better nonmagical weapons and armor, for no real reason.
Pretty much, unless you use the most basic texts available, your game could easily be hosed. The SRD includes a lot of stuff. But you should be fairly safe sticking with the PHB1, DMG, and MM1.
Boring? Too limited? There are other games than these, but you generally lose stability when you introduce more content.
Oh, and consider that R6 d20 variant. Basically, you are limited to 6 character levels. After that, every 5k XP you choose a new feat that you qualify for. If you have a race with a level adjustment, your maximum character level is lower. So a PC with LA+2 can have only 4 character class levels because that puts him at 6.
This means there are simply no 7th level characters. Monsters remain unchanged. Treasure with CL over 6 cannot be created except through the same processes that produce artifacts and relics. So the DM can still put those in, but they're outside the PCs' power to create.
Hirelings and fighting dogs retain their value far longer. The Paladin's Warhorse and the Wizard's Familiar are pretty awesome because the PC never has to deal with a 10d6 Fireball or a 20d6 Horrid Wilting.