Tav - I'm reading through ACKS, and I have to ask... what on Earth possessed you to essentially recreate THAC0??? (that's right, folks...the ACKS attack system features the number "To Hit Armor Class 0") Esp. when (since you've reversed AC), you're so close to the straight-up 3e "to-hit" model anyways? Your system seems to introduced added, and unnecessary, complexity in the form of level-variable TNs (Target Numbers), and I don't understand how this is a benefit.
Nellsir, I'm lead designer on ACKS. The answer to your question is that we wanted ACKS to have a character-centric task resolution system. For each question of "what do I need to roll to succeed" the answer is always found on your character sheet, rather than in the DM's announcement of a Difficulty Class.
A player can look at his character sheet and know what he needs to save v. any effect, hit a standard target, hide in shadows, find a secret door, etc. It's listed right there.
For rolls that are not normally modified much, such as Open Locks, this is clean and easy. For rolls that are often modified, such as attack, it has the pleasant benefit of making the "difficulty" quite transparent.
This puts the Judge (DM) in the position of offering modifiers to those rolls, rather than creating DCs out of thin air. If the Judge says "save v. poison at -20" then there's great outrage - that's well outside of ACKS norms. Whereas in 3.5, for instance, it's just assumed that at high levels you'll face saves of DC25+.
The problem I've had with 3.5/4e style DCs is that they encourage a treadmill where as player bonuses go up, DCs mysteriously also go up, and the chance of success remains the same. This was implicit in 3.5 and explicit in 4.0.
Whether our approach aesthetically appeals to you is of course a different matter, but considerable thought went into the decision. I hope that makes sense and that you'll give the unified "throw" mechanic we've built a try before dismissing it as merely a legacy of 1e/2e.