This is mostly a game of strategy. While there is an element of luck as well there is also the third element. Not everyone is on the same playing field. I remember when we first bought cards back in Revised days. Everyone had a 5 color starter deck. We occasionally bought an Ice Age booster, but we were generally on the same page. That is definitely not true for everyone.
Common cards are important to know because well they are cheap and available. Any of the cards I'm about to list here are playable and generally splashable (and again cheap). Not only cheap but available as these only include the latest block and Magic 2013.
20. Cloudfin Raptor
One of the best one drops in the game, needs many big creatures to be uber effective, but even a 1/2 flyer first turn can be a pest against the wrong deck.
19. Drakewing Krasis
Maybe this thing has killed me too much, but I love it. If you can't deal with flying the 3 damage every turn will get you quickly. Doesn't hurt that he trades with normal 3 toughness creatures.
18. Ivy Lane Denizen
In a mono green deck with many creatures this thing will just give awesome value to your army. Especially if you play multiples.
17. Essence Backlash
I'm not a big fan of counter spells in most decks, and creature specific ones are even worse. This one is expensive to boot. However the damage part of this card can be very useful around the time you could cast this thing. Destroying a 3/3 creature and doing 3 damage is worth 4 mana.
16. Basilica Screecher
Extort is awesome, and flying is awesome. Put onto one creature and you have a valuable little card.
Anything this cheap which gains you life at the expense of your opponent with left over mana is sweet.
Other Common Ideas: Syndic of Tithes, Bloodhunter Bat
15. Sluiceway Scorpion
A 2/2 deathtouch for 4 might make this fairly relevant, remember how much of a pain Thicket Basilisk was? But to add the scavenge ability and you have a great card all around.
14. Ajani's Sunstriker
Good two drops are important and when you can gain 2, 4 heck maybe even 6 life before biting it gives some great advantage.
Other Common Ideas: Boros Mastiff, Skyknight Legionnaire
Blue doesn't get straight removal like White or Black. It also doesn't get damage spells like Red and Black. It does however get cards like these which nullify creatures. There are many similar ones, but this one is definitely one of my favorite as it targets artifacts.
12. Thrashing Mossdog
I'm not sure which ability is my favorite, lifelink, double strike, flying, etc. What I do know is that the scavenge ability just gives so many options and ideally a creature like this already has cost your opponent a card.
All three of these come down to which secondary ability do you want on your creature, reach, trample, flash or flying on Cloudfin Raptor. I like these better than the raptor as they don't require other cards to be effective.
Other Common Ideas: Battering Krasis, Shambleshark
10. Splatter Thug
Awesome version of the Gray Ogre. 3/3 first striker on turn 3 will surely get some damage in.
Other Common Ideas: Goblin Electromancer
9. Martial Glory
Almost a super giant growth, it has such potential to two for one and allow your creatures to live through battle.
Other Common Ideas: Madcap Skills
8. Deathcult Rogue
Unblockable cards can do wonders for other types of cards - infect, equipment, cipher. There aren't too many rouges out there though Dragon's maze did add a bunch hopefully in response to this thing.
Other Common Ideas: Bladetusk Boar
7. Oblivion Ring
White has so many removal options, but Oblivion Ring might be the best as it can take anything out for a mere 3 mana. I did just notice this picture looks gray, so it might not be common?
Other Common Ideas: Pacifism, Divine Verdict
6. Dragon Hatchling
Not overly powerful but in a mono red deck this thing can do some damage. The problem with it being red is that red almost always has something else to do. Just imagine if it was green though.
5. Sentinel Spider
Green finally got it's Serra Angel/Sengir Vampire. There is no argument that this is good value for a common creature.
4. Deadly Recluse
I can't think of many better defensive cards period. And the thing is only 2 mana... it gives Skylasher a run for it's money.
3. Searing Spear
Lighting bolt or Incinerate it is not. But it is still an instant and still a great card.
This thing is annoying as you can't get rid of it. The cards below are good in their own way. Rancor doesn't have the element of surprise, but the re-useability factor makes it one of the best auras and common cards.
Other Common Ideas: Pytoburst, Titanic Growth
Best removal spell for a black deck? Beats Terror, Go For the Throat, Grizzly Spectacle, Doom Blade, you name it.