For me, outperforming your ADP (average draft position) is a huge key as ggs mentioned.
But also, it's about synergy, especially for H2H leagues.
If you try to build a team by stretching for every category, you'll end up with a team that is mediocre in everything and dominant in nothing. I've built teams like this before in the past and even if individually they all have decent fantasy years, it may not translate to weekly victories depending on your competition.
Instead, I try to build teams that are really geared towards a certain set of categories, almost like a "specialist" team. Recently, I won a very competitive keeper league with my friends using this type of "specialist" build.
My roster was:
My team was completely dominant in FT%, treys, and points. In most weeks, they would be very competitive in rebounds, blocks and TO's as well. And during certain weeks, when my team collectively shoots well, they are hard to beat in FG% too. I've completely thrown assists and steals out the window and focused on 6 main categories and remaining relatively competitive in a 7th.
By drafting this style of team, the idea is to continue to augment the strengths that you already so that you can dominate in certain categories. If I had instead replaced Ryan Anderson with another PG to reach for assists and Roy Hibbert with a "steals" specialist, I would not have nearly been as competitive in blocks, rebounds, and treys as I would have been. Similarly, having another PG and a "steals" specialist would not have made me any more competitive in those 2 categories compared to the other teams in the league who were "dominant" in those categories. I would have effectively neutered my team in terms of it's competitiveness by trying to "reach" for too many categories.
Hope this makes sense. Probably one of the biggest flaws that newcomers make when drafting H2H teams.
*rocking the new blamer sig*