As conspiracy-y as this is going to sound, I really believe that they intentionally limit Forge such that the maps it produces are visually unappealing and lack diversity. DLC maps make up the sole post-launch profit source for the Halo games, and a better Forge would encroach on those profits.
Think about it - the Halo map making community is incredibly dedicated. With the ability to do better lighting/objects/environments, etc., there's no doubt that there would be quite a few professional quality maps. Combine that with the fact that user maps tend to enter the matchmaking cycle, and there would be very little incentive to purchase new DLC maps. Why bother paying for a few new maps that not everyone will have when there's a huge supply of free maps that look just as good?
As things have been, every Forge map has been immediately identifiable as a Forge map. It's the same flat, depthless lighting and sea of grey blandness. If I recall correctly, Reach was particularly offensive in this regard in that the grey and flatness was so bad that it actually threw depth perception off.
While I suspect the quality of Forge to improve both visually and functionally, I don't think we'll ever see a point where Forge maps aren't immediately identifiable as such. I strongly believe that it's an intentional design choice to have them that way.
