Bandwidth when you're not adjacent to the router.
I did extensive testing on three different routers in my house - a Netgear WNDR3700, my present TPLINK WDR 4300, and a Netgear AC1200. The latter two I received via Vine, so they were review copies, and the AC1200 was an 802.11ac router, while the other two and the RT-N66R are N routers.
Real bandwidth falls off rapidly with distance and obstacles. You're only going to get 100+ mbps when you're adjacent. Go upstairs and it can easily drop to 20-30 mbps, which is enough to make a serious difference in many routine tasks. In one test spot in my house, the difference was 40 mbps for the TPLINK WDR 4300, 21 mbps for the Netgear 3700, and no connection at all for the AC1200.
From what I've read, the RT-N66R is very good in this regard, though I've never tested one personally.