Thieves in DCCRPG have quite a few innate abilities, most of which are directly drawn from the D&D "stock." For example, consider things like Thieves' Cant, backstab, pick lock, read spell scroll, find trap, hide in shadows, climb walls, etc.; all of the thiefy things like you'd want. In fact, all of the thief abilities, like in AD&D2, are percentile rolls, rather than d20 rolls. In such a d20-heavy system, this seems a bit odd. A few people have commented that this was one thing they didn't like about AD&D, and wished that DCCRPG normalized the system a bit. Personally I am also baffled why you'd need percentile rolls for thievery checks, rather than just a straight d20 check like in D&D 4. Since all percentile values are given in multiples of 5, could someone just set a d20 DC on the roll? certainly. 25% chance turns into a DC of 15; 5% chance turns into a DC of 20. Kill any modifiers on the roll (for now), and you have a straight n-in-20 chance of succeeding just as the Twin Gods
I'm no statistician, but my gut says that the only reason for rolling two dice across a larger spread than one die across a smaller one is that it SEEMS like the d% system would give a slight statistical advantage of succeeding. It might be thought fallacy, so I did an experiment. I rolled a d20 against a DC15 threshold, and d% against a roll-under 25 threshold 30 times, each (successes in bold).
As you can see, in my experiment (I was rolling GameScience Precision dice on a hard wooden table covered in a thin cotton tablecloth), for both configurations I got 8 successes out of 30 attempts or... roughly a 5 in 20 chance of succeeding. DCCRPG DOES concede that the Thief's agility modifier affects the success rate, where every +1 earns another 5% chance to succeed. AD&D2 is much less forgiving. Given the statistical harmony between a d20 roll and a d% roll, I see no reason, personally, to not houserule in substituting one for the other; instead of the agility modifier bonus being +5%, just leave it at the modifier value and add it to the d20 roll. The only caveat to this is that, while most skills improve at a rate of 5% a level, "Climb Sheer Surface" improves at 1%. But, given that it's a skill rated at 90% at level 1, I hardly think an improvement of a 92% chance would mechanically have much different than an unmodified DC of 2.
Another significant difference between AD&D2 and DCCRPG is the Thief ability to read spell scrolls. At level 10, Thieves gain the ability, in AD&D2, to read spell scrolls at a 75% accuracy. Failures result in the spell backfiring. In DCCRPG, Thieves gain the ability to attempt reading spell scrolls at level 1, albeit with an almost impossibly high rate of failure. Good and Chaotic thieves can make a spell check, but must use a d10 for the check die; given that the spell check DC is 10+(spell level x2), a first level Thief would not be able to cast a first level spell from a scroll, since their maximum roll would be 10, and the minimum DC is 12. It does say that clerics add their Personality modifier, and Wizards add their Intelligence modifier to spell check rolls, but it does not say that Thieves add any modifier to spell checks, so therefore though the possibility exists, and considering that there do not appear to be 0-level spells in DCCRPG, it is actually impossible for a first level Thief to succeed on a spell scroll. This quickly changes, since by level 5 Thieves may use a d14 (usually) to attempt a spell check. Neutral Thieves apparently have an easier time casting from scrolls, because at level 1 their check die is a d12, and by level 5 they have already advanced to a d16. The pattern set from the level 1 to 5 table is every other level the check die improves, so at level 10, if the pattern continues, the Good and Chaotic Thieves would be rolling a d20.
Backstabbing is not particularly different between DCCRPG and AD&D2, despite some apparent opinion to the contrary. The only rule is that "when attacking a target from behind or when the target is otherwise unaware, the thief receives an attack bonus." This is actually more generous than the AD&D2 provision, which also states that the victim must be humanoid. Additionally, AD&D2 only has a damage multiplier for backstab successes, whereas DCCRPG automatically awards a critical hit. Critical hits reward a roll on the crit table, which can occasionally be very damaging, but also can be uneven. The luck of the dice can mean the difference between an additional 3d3 or 2d4 damage, or a result of "Foe is reduced to making wet fish noises" without a real benefit (except, perhaps, that it can't call for help?). Being a longterm fan of MERP, rolling critical hits on tables is very appealing to me, but I can also see why many people would just want an additional attack die, or a damage multiplier. It's hard to hit with a backstab in DCCRPG; but so it is as well in AD&D2.
The last thing to discuss regarding the DCCRPG Thief is the issue of Luck. All DCCRPG characters have Luck, and all can burn Luck points (permanently) in order to avoid something particularly sticky from happening on a failed or botched roll. Thieves luck out (heh heh) on this one: they are the only class that can (slowly) recharge Luck points; evidently to be more in accorance with their Tricksy™ nature. Additionally, the Thief gets to add a die roll modifier to checks when burning luck, instead of just 1:+1 like for every other class.
Besides all that, the D&D Thief and the DCCRPG Thief are more or less similar. Overall, I like the DCCRPG changes to the Thief, and I think it makes the class more playable and more interesting.