Well, there's a lot of answers because lots of people find different things broken:
I'm copying & pasting from my house rules document I keep at G-Docs...
TOO STRONG:
Warforged (our fix: no embedding components, must make death saves as normal)
Fighter: The Tempest Fighter technique and the Battlerager Vigor Fighter class build options are banned. Both of these are insanely powerful. Tempest has damage on par with a TW Ranger, but has the benefit of better AC, more HP/surges, and can mark. BRV fighters generate so many temp HP that they are nigh-invincible at times (most of the time, in fact, and they give up very little to do so).
* The Invigorating and Rattling keywords still work as described on MP 7.
Wizards:The Orb of Imposition class feature is banned. O-R-B spells broken!
Penalities and Bonuses
1. All penalties and bonuses will be typed by default, unless specifically ruled to be untyped. There's a lot of weird stacking issues going on that makes things better than they should be. This helps keep things sane and allows access to more magic items by default.
2. The Penalty and bonus types and when they apply are as follows are available in the obscure rules document. In general, know that powers (from class, race, etc.) grant power bonuses (or penalties) and items grant item bonuses, and feats grant feat bonuses, and bonuses or penalties of the same type don't stack.
3. For now, feats will use the bonus types (or untyped) as indicated in the feat, but all feats that impose a penalty use a feat penalty type.
4. Magic items that grant a bonus to damage grant an item bonus (including changing a bonus from a different specified type) unless specifically ruled otherwise by the DM.
Minions:
1. If a minion is damaged by an attack roll which hit (the minion does not need to be the target of the attack, but the target must otherwise be legal), it dies. (Example: Cleave (Fighter At-will 1, PH 77) deals damage to an adjacent target equal to the fighter's STR modifier if the attack hits. Since this damage is only applicable if the attack roll hits, the damage kills the minion.)
2. If a minion is damaged by any means other than #1 above (e.g., automatic damage from a flaming sphere, aura, etc.), it is bloodied. If the minion is already bloodied, it dies instead.
3. "When you kill an enemy ..." or "When you bloody an enemy..." powers or abilities do not work on minions, except as noted below.
4. When you kill a minion, any player with a "When you kill an enemy ..." power can roll a 1d4. On a 4, the player with the power or ability may activate it normally.
Powers in general/wording issues:
1. "An attack" is considered the entirety of a standard, move, minor, immediate, or opportunity action, no matter how many attack rolls that consists of. (This typically matters for powers that give a bonus "to the next attack" or for "an attack that does not include ...")
2. Rerolls (such as those granted by Elven Accuracy or Second Chance) come with all the bonuses and/or penalties of the original roll, plus any additional ones that apply to the reroll.
Specific powers
1. Righteous Brand (PH 63) uses your Charisma modifier to determine the power bonus. Also, the power bonus only applies to the next melee attack made before the end of your next turn (instead of all melee attacks).
2. Dual Strike (MP 7) is banned. See notes on Tempest Fighter, above.
3. Storm of Blades (PH2 56): change "against the target or against another creature" to "against another creature within reach" and "your Constitution modifier" to "half your Constitution modifier". Repeat after me: attacking 8 times with one encounter power is not kosher.
4. Hurricane of Blades (PH2 60): replace "five" with "three" and add ", no more than two attacks per target." to the end of the Effect line.
Specific items:
1. The Rod of Corruption (PH 239) does not count as placing your curse and/or cursing any of the enemies affected by its property. They count as "cursed" by you for all other effects and purposes, however. Unlike WotC, we saw that one coming.
2. The double sword (AV 9) is not a light blade. Bad rogue, no cookie.
3. Bloodclaw weapons (AV 65) only work at a 1:2 ratio, never a 1:3 ratio regardless of number of hands. The power also applies only to the next attack roll (this is an exception to the general rule of "attack"="power") made with the weapon, not to all attacks you make with a power, such as with Twin Strike, etc. You may choose before each attack roll you make to apply the power (taking the damage to yourself each time) or not. Too much power for too little cost.
4. Mounts are banned. Too much headache. If the DM gives you one as a plot device, which he will almost certainly regret, you may use it.
5. The Cloak of Distortion (AV 151) is banned. If you have to ask, you obviously don't understand the system. [We're recently considering "grants you concealment against ranged attacks from more than 5 squares away"]
6. Grasping Weapon (AV 70) only works with melee attacks. Once per encounter pull soft-squishy enemy into a maelstrom of your allies is not nice. Consider a Transposing Weapon as a higher level substitute.
7. All non-neck slot items, except for masterwork armors, which grant a static bonus to a non-AC defense are banned. Your defenses will do fine with the free feats we're giving.
* Items with encounter/daily properties or powers in addition to static defenses bonuses will be available without the defense bonus at a level determined by the DM.
WATCH LIST FOR THINGS WHICH MAY BE OVERPOWERED
This section describes powers/ rules / items that we are currently monitoring closely for balance, and serves to give players fair warning that building your entire character around a trick in this section may be a bad idea.
* Reckless weapons will probably see some sort of change to reduce their power level. At least at Paragon and Epic. Or make them more reckless.
* Bloodclaw weapons may be banned outright if even 1:2 proves overpowered
* Righteous brand may still be overpowered.
* Spitting Cobra Stance (MP 47) might be restricted to "make a ranged basic attack as an immediate reaction..."
* Versatile Master (PH2 193) is broken. If WOTC does not fix this feat before paragon play, it will require Paragon Multiclassing for all benefits.
* Any untyped bonus -- it may become a typed bonus between any play sessions.
* The minion rules are on the watch list. Possible changes include: automatic damage never damages a minion, or we never have minions at all, the minions all turn into dragons and eat you, or something else entirely off-the-wall.
TOO WEAK/POOR DESIGN:
Minotaur (str and con both help fort, so the race should get a +1 ref or will, we chose race; also the goring charge encounter power is very weak, we changed it to be a minor action / special: you can take additional actions after this charge)
Expertise feats (feat tax = not fun, we give everyone an untyped +1 at 5th/15th/25th and ban expertise feats)
Defense feats (we give everyone paragon/robust defenses free, and your choice of one of the epic defense feats at 21st, and one more at 25th. You can take the third if you really want it, but most characters won't need it)
