Is there a limit on capicity or evil features on a shotgun?

Semi-automatic = yes
Pump = no (except barrel / overall length)

27CFR 178.11 Title 27, Volume 1, Chapter 1, PART 178, Section Sec. 178.11  
(assault weapon defined)
(d) A semiautomatic shotgun that has at least 2 of--
   (1) A folding or telescoping stock,
   (2) A pistol grip that protrudes conspicuously beneath the action of
the weapon,
   (3) A fixed magazine capacity in excess of 5 rounds, and
   (4) An ability to accept a detachable magazine.
   
   Semiautomatic shotgun. Any repeating shotgun which utilizes a
portion of the energy of a firing cartridge to extract the fired
cartridge case and chamber the next round, and which requires a separate
pull of the trigger to fire each cartridge.
   Short-barreled shotgun. A shotgun having one or more barrels less
than 18 inches in length, and any weapon made from a shotgun, whether by
alteration, modification, or otherwise, if such weapon as modified has
an overall length of less than 26 inches.
   Shotgun. A weapon designed or redesigned, made or remade, and
intended to be fired from the shoulder, and designed or redesigned and
made or remade to use the energy of the explosive in a fixed shotgun
shell to fire through a smooth bore either a number of ball shot or a
single projectile for each single pull of the trigger.    

Thanks.

Ain't it great that the semi-auto shotgun you bought for defense can't be equipped with an extended feed tube or pistol grip?

But it's OK for a pump action -- which is more reliable.

(Don't get me started!)

Best friend has a Mossberg 590, ghost ring sights, folding stock. An awesome piece of machinery!

Actually you COULD have a semi-auto shotgun with a feed tube larger than 5 rounds OR a pistol gripped stock.
Two makes it a "defined" assault weapon, one is OK.