I hate to say this, but at some point in circuit density, a multi layer board is indeed the way to go. Stuffing an unbroken ground plane under everything helps a number of issues. It's not going to improve the cost side of things any. Everything I do these days at work is multi layer....
The ARM's operate at higher speeds internally than most of the older MCU designs. That means faster edges coming out. Faster edges always will mean more "stuff" crawling all over everything. Bypass caps and decoupling resistors tend to multiply because of this. I'm not real sure that the Due boys have come completely to grips with all of this. That puts an even bigger burden on a shield board like this one.
The ARM's operate at higher speeds internally than most of the older MCU designs. That means faster edges coming out. Faster edges always will mean more "stuff" crawling all over everything. Bypass caps and decoupling resistors tend to multiply because of this. I'm not real sure that the Due boys have come completely to grips with all of this. That puts an even bigger burden on a shield board like this one.