Icon usage

Usually a "shield" is for "defense". Maybe for a "Champion" two crossed swords and for a "Normal" unit only one sword.

Strength = a Fist (like http://game-icons.net/skoll/originals/fist.html)
Durability = sounds like "defense" so I would suggest a Shield (like http://game-icons.net/sbed/originals/shield.html)
Intelligence = a Open Book (like http://game-icons.net/lorc/originals/enlightenment.html)
Agility = a Bulls-eye (like http://game-icons.net/skoll/originals/bullseye.html)
Not sure about "damage". I would favor an "explosion"-type icon. Or a spinning sword like this: http://game-icons.net/lorc/originals/spinning-sword.html
Or a pointy sword like this: http://game-icons.net/lorc/originals/pointy-sword.html

Something along those lines... Also check out "Game Icons.net" for a huge selection of FREE icons for any type of project...

+1 to gameicons.net

The noun project is also very helpful for putting pictures to words. Even if you don't use their icons it can get you thinking in new ways:

https://thenounproject.com/

Just a thought, but in recent years we've seen iconography expand considerably... for example, you don't have to go with just a pair of wings for Agile (even though I think this was a fantastic suggestion), you can have a little man leaping with action lines behind him; a champion can be a roaring maniacal warrior with two axes, etc...

Think of the Iconography in the emoji section of your phone, or the ones used for abilities in World of Warcraft. Better yet (for board game purposes) Lord of the Rings LCG -- they have a little mountain with a bird flying and a river running through it, all in one little icon circle; and most of them are very clear because of the stark black and white contrast. You can add a single color to each icon, instead of white, and they'd be obviously differentiated.

If you have knowledge of photoshop or illustrator, you can make elaborate icons like these from almost any picture, and you can probably also find a lot of things like this on the web.

Best of luck to you!
B. Brian Blackwell

I kinda have to say something about this.

On one hand I will agree that icon clarity has improved over the years...Photoshop, Illustrator, and programs like them are definitely allowing for more diverse icons. The example of LotR LCG is a great example.

However, speaking as a design instructor, iconography is not about what you can put in the icon as much as what the icon communicates. A wing symbol for Agility is STILL a wing regardless of whether it looks like feathered detailed wing from a Renaissance painting or whether it looks like a fat, puffy wing from Mario Bros. With emoji's, a detailed emoji of a smiley face still communicates when I do this: :)

You also have to consider what the icon is used for and how it's going to print. The detailed icons for LotR LCG are really for indicating expansion sets; those icons have no real practical (in game) purpose other than differentiating themselves from each other. If you're putting the icons on a card, it has to still READ. In fact, the more detail a card icon has, the potential to miscommunication rises sharply. The point is, card icons...especially if they're going to be as small as Magic: The Gathering land icons need to be simple and readable.

Here's an example. For the longest time, I thought the Paint Bucket tool in Photoshop was a graduation hat. I know I'm not alone.

Simplicity is the key. I know that using the same shield icon for armor is boring and repetitive, but it's simple, it communicates, and is effective. If you want something different, you'll have to research and test with people, which is a work in itself. When I tried looking for alternate "armor" icons, I tried using a brick wall pattern, a full suit of armor, a plate with rivets on them, many things. Ultimately, it was time I could have used in design. Creating effective icons is HARD, don't overcomplicate yourself. If the shield communicates armor, then keep it as a shield...try a variation of the shield look instead of looking for a new one.