Flavor Kills

Keep in mind that when you build a deck, you can do one of two things:

Either build a deck focused around the 'flavor' (maybe based on the art of the cards or the cards' part in the storyline) or you build a deck that is for more competitive play. It can be either one, but not both (unless you're making a deck around Ravnica's guilds, in that case only, yes you can choose both).

It's hard to make an example for you to understand unless you actually try it yourself. A kind of vague example is building a deck around Glissa, the Traitor. Going all flavor would be filling the deck with cards from around her expansion, which would make sense since her card works very well with artifacts, but get too consumed by that fact and the deck would become unstable and weak. Too many artifacts might not be too good.

-MT.