Whenever I am selecting cards for a deck, I use a website like magiccards.info to search. Something as simple as "draw a card" shows up in every color and in lots of different ways. Certain cards rise to the top depending on what I am trying to accomplish with the deck, the theme, and how I think it will play when it is all put together.
For example, if I wanted to put a deck together for a constructed tournament (like Legacy), I look for cards that give me the biggest effect for the least amount of mana. The priority is to get the effect for the least amount of mana. In Commander, the priority is the other way around. I still want the card with the biggest effect for the least amount of mana, but in Commander, I the biggest effect is the priority.
Here's a simple example of how this works. Let's say that I am building a quick, Black / Red deck for Legacy and I want to include some burn. For one red mana, Lightning Bolt is an instant-speed, 3 damage spell. Boom. At one Phyrexian red mana, Gut Shot does its thing, but it only does 1 damage. A while back, they slowed down the direct damage by printing Shock for one red mana and 2 damage. Shock is inferior to Lightning Bolt. They go back and forth on Lightning Bolt and Shock depending on the set, but one of those two spells is usually the benchmark for the mana-to-damage ratio curve.
Now, let's say that I am building a Black / Red Commander deck. Do I want Lightning Bolt, Shock, or Gut Shot? I probably don't want any of these spells because even though they are efficient, they don't do much in the grand scheme of things. In a game that can last more than 10 turns with a whole table full of players, 3 damage isn't going to cut it. Besides, there's a really good chance that I'll have the mana to support spells that cost way more than one red. So, what I really want is a spell that hits everyone or everything at the table for lots of damage at the lowest mana cost, but the priority in this format is the damage not the mana.
So, a spell like Disaster Radius for 5RR (7cc mana) costs a lot more than Lightning Bolt, but the effect can really swing the game. Sure, you can't drop Disaster Radius out of nowhere to burn a little creature that is giving you trouble, but you can drop it out of nowhere to really flip the game in your favor. How about each of your opponents creatures take X damage?
Another way to look at this is in one-vs-one constructed formats, turns 1-4 really matter. Games are often decided in those turns. Decks that hold out and take a long view of the game can work too, but you're still only running up against one opponent. In Commander, you're looking down a lot of barrels and turns 1-4 are almost always just setup turns. You might play a ramp spell or two. You might play a small utility creature or something that gives you a small incremental advantage. You probably aren't coming out guns blazing and dropping bombs all across the table.
By turn 5 though, things get interesting. Many players will have access to their Commander and a good amount of mana thanks to all the land dropping and ramping from the first 4 turns. But, you still probably aren't going to cast a flurry of spells that turn. This is where looking at your spells as bigger effects starts to matter. All the way up to turn 10, you are going to have enough mana to play one or two spells per turn. So, if I have 7 mana available, I want a big spell, the biggest spell I can get and still cast it. Bolting someone for 3 and passing my turn is not what I want to do here. Pointing a Flame Wave at someone looks pretty good though.
