So, what is the problem with the thief ? Well, simply put, except for climbing, they suck at about everything thiefy. Which meant, under ODD&ADD1, to have any hope of not getting your hand cut off the first time you try to pick a pocket, and get caught, you end up needing to play a non-human thief with an 18 dex. They do excell at backstabbing, true, but just try and get into position by being bold....or moving. No, you pretty much have to mail yourself to your intended victim and hope that he is facing the other way when you burst out. You could climb a wall, and wait for someone to walk below you, I suppose (piercer costume optional), and hope that they aren't looking up, cause with a 10% success chance, you are not going to be hidden; and with 1d6 hit points, you ain't gonna get as second chance.
See, the problem isn't that the thief broke D&D, it's that the thief is broken. Its a great addition to D&D conceptually, and in keeping with a start small grow tall approach to character development, but it seems like it is the only class that got saddled with a truly untrained first level. Look....the fighter can hit a random peasant 50% of the time, right ? The Mage can put him to sleep.....the cleric ...well, strictly speaking the cleric can't cast a spell, but he still has a good chance of smashing his head in or, even better, if its an undead peasant, making him run away. The thief ? He can...ummmm....hide from him one time in ten; or alternately, sneak past or pick his pocket one time in five. He could, to be fair, hear him pretty well.
So, the solution we used back in the day was this: a first level thief started with a base chance equal to 30% +/- 5% per dex point above 14 or below 7. Then, simply add the greyhawk percentages at each level, incldung the first. A non-thief could try any of them, but only using one half the base number. A thief using the wrong armor simply lost all this level bonuses. Done.
It gave a nimble thief about a 50/50 chance of success for most thiefy things - not great, but way more in line with the other classes, who are moderately pathetic at level 1, as well.
Nowadays, for the basic thief skills* I use a modified quickNez stat check:
4d6 roll<=dex+level to succeed.
Works great, and the 4d6 bell curve is a nice change that allows higher level or more nimble thieves less variability than (say) a d20 roll. I still use 1d4 for HP - one should be able to clean the slimy little sneak's clock, after all - possibly what made him turn to theft in the first place....and so the great circle of life continues.
* picklock, disarm trap, pick pocket, sneak, hide. I exclude climb as a no fail with the right tools; and search as it makes for lazy players -which I note isn't something a thief does in OD&D anyhow.