I would think that it's not always best to always have the fastest specs in products. Besides, AC is still new and not native to most client HW. And I don't think it's been certified as of yet either. I would not jump to conclusions until it comes to market. The DGL-4500 didn't have the best in Wifi for what it was, it could only due one or the other bands on WiFi unlike the DIR 8 Series routers do. I presume there was some reason behind that as I presume there is probably a good reason behind the 5500's specs. Yet, the 4500 is still a great gaming and general purpose router. I speculate that one possible reason is that they didn't want to over tax or under perform what is supposed to be a "GAMING" router so they are putting more into the gaming aspect, form and function rather than going all out and putting the most WiFi. It's not always needed that you technically need the highest WiFi connection. Besides, the speeds are only between the router and connecting devices. Not data flow. Were just getting bigger pipes for data flow. I presume D-Link is attempting to balance HW, design and performance to make this a great gaming router.
Just having the most or highest specs out there doesn't mean things will be great. Honestly, having AC1900, whats available for it right now and what will you really need it for? AC still needs to be finalized and certified as well.
So far I'm not highly impressed with AC. Still needs work.
My 2 cents.