Itemization: One thing that I constantly hear from high elo players is that they know how to itemize properly for a map like Dominion. I feel that a lot of people, in all game modes, can only build in two ways, glass cannon or pure tank. It seems almost impossible for players to find some sort of balance between the two. In Dom, being a glass cannon means you will die before doing any damage. If everyone builds tanky, then the team has no damage to fight with. This applies to both AD and AP champions.
Jungle Control: I often love to draw parallels between SR and Dom. In SR, everyone knows what to do in the laning phase. But it seems like as soon as lanes break, everyone is confused and do whatever they feel like. I have spectated enough SR games to know that a team that had an obvious advantage during the laning phase had the tables turned on them during mid-late game. I am convinced that a lot of the high elo Dom players who decided to play ranked and made gold/platinum/diamond ranking probably had less than perfect laning phases but their knowledge of what to do during the mid-late game stages of the game is the reason they could turn even an unfavorable situation around.
In Dom, the Windmill fight is similar to the SR laning phase. People know what to do during the laning phase of the game, but as soon as the windmill is captured, people run around not knowing what to do. I have talked to several people (mainly you), who talked about superior jungle control. I watched enough CaptnBuwBerry streams to know that whenever you play, you constantly remind everyone of jungle control. You would not be stressing this if this was not important.
A skill that is incredibly important in all game modes, but one Dominion stresses the most due to lack of ward/altars that give us at least some idea where the enemy champions are.
I also know that Map Awareness also ties into superior jungle control as well.
These are the three things that I believe a lot of players (including myself) are lacking and preventing us from advancing to the next level. I mean, there are more reasons like spamming OP champions, but I believe the three I listed are the three most important ones.