Novaing and optimizing follow the same path.
Sharp shooter, GWM, smite, action surge, eldritch blast are the main tools for both.
SS and GWM have been discuss enough.
MC abuse have been discuss a lot too.
MC can produce quadratic like effect. An Ek who take 2 level of paladin could at high level use an impressive number of smithing dice,
A wizard with 2 level of Fighter can use 2 high level spell in a same round. The more the wizard level increase the more this effect is important.
Simple nerf could be applied.
Smite can be limit to 2 time per round, 3 if you want to include a reaction. A full paladin wont see much difference, but it wont be combined with smite,
Action surge can be limited as well. At level two allow only one additional attack. Allowing multiple attack or cast spell only at level 5 fighter. Again a full Fighter wont see much difference.