The rules were based on DPG's universal task profile switched to d10 from 2d6. The original rules had the oddity that the d10 ran 0-9 instead of 1-10, but this was changed in the 2nd edition. They work quite well, but I'd argue 2d6 worked better.
The combat rules were vastly improved over Traveller, and separated the probability of hitting and the chance of armour stopping the hit. I would note that when Hanrahan wrote the Mongoose Traveller rules the one thing that was done really badly was the armour rules. Mongoose Traveller could benefit from taking chunks of the 2k3 combat rules.
T:2k3 was lacking a bunch of stuff like XP and skill increases, but these were added in 2k3. I'd argue that not being able to "level up" is actually really realistic.
Ultimately, the 2k3 rules were the first step towards the GDW House System of the 1990's, for good or ill.