They have a bit of a nutty-grainy flavor. Biscuit/Victory malt and a little chocolate malt help bring this out. Some of my favorite recipes also include Special B, and some include brown sugar or molasses.
Nut browns usually have low-to-moderate hopping with English hops, where American browns are hopped more aggressively with American varieties.
That said, there is no BJCP definition of the style, and they're considered either Southern- or Northern English brown ales. Northern browns are a little hoppier than southern. So if your beer tastes a little nutty/grainy to you, you can call it a nut brown if you want.
One of my favorite recipes is from Norther Brewer. Basically, it's 6lbs LME; 4oz each of chocolate, Special B, Bisciut and Special Roast; 1oz fuggle. This makes a beer almost identical to one of my all time faves, Wachusett Nut Brown Ale.
Stop using so much caramel malt. Your beer will thank you.
(yes, Carapils is a caramel malt...so is Special B)
pujwI HIq Mild Ale
KPA Khitomer Pale Ale
Quadro (quadrotriticale ale)
Beer of Honor
Black Bat'Leth Stout
Targ's Blood Nut Brown Ale
Supersecret sour beer experiment v1.0