I've learned these over the years:
Beginner: Assemble in white heat, cut in cold blood. Be excited when building a deck, add all the splashy effects and pet cards that your heart desires. Then when it's time to trim it, do not let emotions cloud your judgment. Be ruthless when deciding if a card deserves a spot or not. Resist the temptation to keep your casual deck at 62 cards just because you have two pet cards that you want to keep as your "signature cards" despite being woefully underpowered or inappropriate for that deck. Conversely, if you really want to keep your signature, then don't be afraid to cut something else, and be willing to live with the toned down power.
Beginner: Avoid compromising lands. Stick to a healthy number of lands in your deck (20-24 in a 60-card deck minimum recommended, 33-38 in EDH minimum recommended, depending on curve) Want to squeeze in that one new card from the latest set? Too often you'll be tempted to just cut a land for it. Resist the temptation and look elsewhere for something to remove. Find yourself going for multiple mulligans a game? Maybe you need more lands.
Beginner: Avoid. Compromising. Lands. The better quality your lands (ETB untapped / provides multiple colors / extra tricks) the better your deck will be overall. Find yourself going for multiple mulligans a game? Needing extra turns to drop something that you could have dropped earlier if your lands had entered untapped? Maybe you need better lands. A twist on this applies to Feather EDH, as you really want as many lands that can produce colored mana, sometimes at the expense of taplands. But the tip holds true: better quality (in this case colored vs uncolored/utility lands)
Intermediate: Include ramp and card draw. Specifically, include better quality or more deck appropriate ramp and card draw. Mind's Eye and Staff of Nin provide card draw, but if you are running a faster deck, they come down too late to really help you. Conversely, a deck with a ton of ramp and affinity probably doesn't mind the high CMC of those two artifacts.
Intermediate: Be sure you have enough gas. Feels really bad when you get to turn five or six and basically draw-gounless you're blue and it sometimes comes down to you adding too many utility or conditional cards and not enough wincons.
Intermediate: Look at how many cards interact with (or depend on) one another. You probably should cut Angelic Accord if you only have seven sources of lifegain in your entire EDH deck... The same applies to all conditional cards. Some may look great, but when you look at the numbers, they may be too conditional and be dead cards a majority of the time. Cut them for something more impactful! (This is similar to u/Itsaghast / OP's tip and I wholeheartedly second it. Will really improve you as a player.)
Advanced: Have multiple and redundant ways to win. This is probably not an advanced concept, but the ways it can be achieved seem pretty advance to me as I only recently learned how extreme this can get when I studied cEDH and saw the myriad of ways that those types of decks can win, and at different points of the game, as well. Stuff like Vivien-Emiel-Village-Bell-Ringer, to flicker effects and mana generation and Blind Obedience, to something as silly as Kamahl and Elesh Norn.
Finally, without any rating:
Adjust to the local meta. Don't be too rigid. Not everyone has the same Magic experience, and while archetypes are there for a reason, you should always adjust and fine tune your decks to suit your playstyle and your environment.