I personally have thought for years and years that the D&D concept of Armor Class was antiquated and heavily flawed, so in this case I believe they are fixing something that is broken. That's just my personal view though, I'm sure some people think the D&D Armor Class system is perfectly fine, they're entitled to their opinion.
I am not claiming the old system was perfect. But I believe that I much rather see Obsidian focus their limited resources on creating stories and content with the old proven system then trying to dissect every piece of game mechanic and reinventing it. I would much rather see them operating under the old constraints trying to build the best (new 2014 version) IE game possible.
I would hate to look back at project eternity thinking that we didn't learn anything from Masters of Orion 3.