Weight lost
It is extremely easy to waste weight (inefficient builder), and every skilled enough builder will tell you that. Often, this sort of criticism is given to Newbies in their showcases. Some builders are particularly difficult on this, like Naryar.
STOCK
-Ballast (Unless used for creating better wedges)
-Heavy weapons like the Sledgehammer. Weapons heavier than 12kg are generally deemed as worthless in stock.
-Any 4WD drive that is not HPZ
-A weapon that's not going to touch the opponents
-Pinks are weight wasted on anything with more than 2 spin motors or heavier than LW.
-Big wheels (GrannyGuard Wheels should be big enough for all your needs. Try to always use Shiny Hub Wheels if you can.).
-Steel armor (Or titanium armor. Always use DSA if you can, and plastic if you need the weight).
-Bad use of extenders (Using a lot of angle extenders can really eat up a ton of weight. Try to use a minimum of extenders when building, all having the same HP.).