The Value of Dominion Games
Dominion is not generally considered a highly competitive game mode (Though some take it very seriously!). Most view it as the land of OP champions and game imbalance. There are legitimate complaints to be had, but Dominion teaches things that other game modes simply do not. When you read the questions people ask pros, you wonder why they are asking things like "What are your runes? Masteries? Build order? Item build?" A lot of these things change based on match-up, based on how the game is going and what their role in the team is. Then there is a strong focus on the laning phase. While laning is no doubt important, I am not sure that is where people should be putting their energy into improving. The biggest difference between high ELO and low ELO is not in the laning phase. I am sure they are better, but they know how to take the advantages they get and turn it into a win. Or even know how to get ahead in the first place. But do you know how to play when everyone is equal on gold? How to counter build? Skirmish? Resource allocation? Group up and fight together? In a game mode like Summoner's rift, there are a lot of things going on at any given time. So if you want to focus on one aspect, it can be very difficult. Thankfully Dominion is here and even with the issues it has, there is a lot for you to learn.
Laning Phase is not the End-All, Be All.
There is a massive focus on the laning phase amongst the general community. During this phase of the game, things like masteries & runes are important. Perhaps that is why I see so many people asking about them when pro's pick a champion. Anymore, these things mean less to me. When I play against a Nidalee, I grab my HP5 page, go into defensive tree for less auto attack damage and pick up an early dorans shield to help deal with the constant harass. If it is going to be a more aggressive, kill you or kill me match-up, I might go more into the offense tree. So asking them about such things is pointless. Their answer will likely be "it depends" and I learn nothing. Further, pro's do not always win their laning phase. How many streams have you watched, saw the streamer lose their lane, but still somehow win? The real strength comes not in winning their lane, but in finding ways to come back from behind, or win from behind. After laning, all that planning for the laning phase means less and other factors become far more important. These things dominion can help teach you and you do not get the noise from the laning phase.
Fighting on an Equal Playing Field
It is easy to play a champion when you have 5 kills and a lot of CS or when the jungler ganks for you all the time. But try doing that well when the enemy has just about as much money as you and your not getting jungle help. Now the game is not about feeding, killing minions, but playing as well as you can to control objectives. One bad death and they could take a tower and your suddenly losing. You have no one to blame but yourself here. Sure teammates make a difference, but I have seen individual players make a world of difference, especially if your the guy at bot. The game is different and you learn how to spend your gold wisely to keep yourself relevant or survive what is happening. It isn't about getting ahead here, it is about doing as much as you can with what you have and the enemy has just about as much as you.
Dominion has a lot of viable champions and the steady exp/gold from the map lets you try out champions in a way that Rift simply does not allow. There is no worry about feeding, you can try out builds here to your heart's content. You also get into a lot of combat, making you learn your champion's abilities quickly or you will just end up spending your time looking at a black and grey screen. There is no ranked here, enjoy playing a champion. I personally take new champions into dominion for a few games before I worry about playing them in Rift. Want to try out something? Dominion is the perfect spot.
Fighting in small quick engagements is a lot of what Dominion is. Often times in Summoner's Rift, skirmishing is what can really get you ahead. Learning the proper timing of when and if to fight can be the difference between feeding the enemy and getting a double kill. One of the best ways to learn anything is to practice it and dominion can give you four times the number of skirmishes that SR does, but without the noise of laning or gold differences.
I was playing Jax top lane and got ganked by Olaf, Lee Sin and Mundo. With only a sliver of health I managed to escape to our Ahri. Ahri realized their health, knew my damage output and knew they had blown a lot to get to me. Time to go all in 2v3! She walked away with one kill and I walked away with two. The kind of experience to notice all that can take time to acquire and requires going through a lot of fights. Dominion will give you more fights for your time than SR to help you gain experience.
Dueling and Counter Building
Dueling and counter building is a basic part of Dominion. Especially when you play bot lane. Pushing is effective, but being able to straight murder your opponent is more effective. You can really learn the limits of a character fighting till one of you dies. It also teaches you counter building. If I am playing against Xin and I cannot out duel him, I can buy a thornmail and start to equalize the difference in raw dueling power. If he continues down his normal build path focusing on more damage, he will eventually hit my armored wall and just die. He could still kill me later, but I have forced him to adjust to me so that I gain the upper hand, even if for a bit. But while I have that advantage, I can press it to turn it into objectives. Also, if a single enemy cannot handle me, it forces them to send more to me, letting my team gain an upper hand somewhere else.
I had one game where I played against an entire enemy team of physical damage dealers. I laughed my ass off as I built Randuin's Omen, Frozen Heart and Thornmail. With a Wit's end for ASPD on Warwick. NO ONE on the enemy team bought a Last Whisper (this was S2, but even a BC isn't enough with that armor) and I won a 3v1. It was, by far, one of the funniest moments I have had in this game. Apparently no one thought adjusting their builds to meet a 300+ armor Warwick was a good idea. No Riven, more AD does not make you hurt me more.
Because of the highly combative nature of dominion, you really cannot be resting on your laurels in item selection. It never ceases to amaze me how many players do not buy armor/MR when the enemy team has all of the same damage type, or keep building like they are ahead.
Map Awareness and Assualting/Sieges
This is a skill that everyone should get better at and I think Dominion is a great place to learn it. It isn't a tough skill, you basically adjust what you are doing based on enemy movements. But I cannot tell you how many people fail to realize this. For example, if 3 people of the enemy team are bot at 30:00 minutes in and Baron is up, why in the hell aren't you forcing it? Same idea here, if 3 people are bot, then push and take another tower while they work on your bot after ganking. It is not rocket science, but it feels like it sometimes. Dominion is all about moving around the map to secure objectives and makes you move quickly to adjust to enemy movements. It also teaches sending the RIGHT amount of force. Just because Lux is trying to backdoor a tower, does not mean you send 3 people. Jax can probably scare the little girl off by himself. I truly feel like dominion forces so much strategy and team work down your throat, you cannot help but improve in areas that are often overlooked in Summoner's Rift.
There Are Many Right Paths
The path to getting better is a multi-pronged path. One way to get better is to just play Summoner's Rift all day. I personally prefer a targeted approach, playing different things to work on certain skills. I believe that Dominion can teach very different but valuable skills to be used later in Rift. Those varied experiences help me to improve and give me a new experience. Sounds like a win-win to me.