First of all, the mahogany. I absolutely love the lush, velvety rich tones these 15 series guitars create. The low end response is so broad and warm, I find it hard to go back to spruce tops.
Second, the finish. Pictures do not do the burst justice, and if you are in the market but don't want the Retro (WHY?), call them up and order one with the burst finish. It makes an otherwise flat finished top glow with a really gorgeous depth.
Third, the Aura +. Fishman really worked some voodoo into this system. As good as the under-saddle pickup is, compared with the images included with the Aura + system, its as quacky as any acoustic pickup. I leave it on preset 2 and forget it, but it really does fix any sort of issue you've had with amplifying your acoustic instead of using a mic.
Fourth, the neck. This wide, 1 3/4 neck is the bees knees I tell you. fingerpicking feels comfortable and easy, and strumming suffers no drastic change in feel or response. The taper on the neck is low and fast, perfect for sliding around as well as reaching those oddball chords. I can't say enough about how comfortable it is.
Wrapping it up: A 00 Martin with a huge, rich, amplified sound, delicious wood, and a gorgeous finish. Buy this guitar, and it will be your go-to acoustic in almost any scenario.