When I am building a deck, I am usually looking to accomplish or prevent a specific thing. One of the things I really enjoy about Magic is that if you know what you want to do, or how you want to do something, there is usually a card or a combination of cards to do it.

Let's start with the D'Avenant Archer and the Heavy Ballista. The Archer (a 1/2 creature, costs to cast and does one point of damage to a target attacking or blocking creature if you tap it. The Ballista is a 2/3 which costs to cast and does two points of damage to a target attacking or blocking creature for the mere cost of tapping it. I built a deck around those two cards; it was not a pretty sight. Ulrich really thought having three Ballista and two Archers out at the same time was a bit uncalled for.
—Helva

I had a Shivan out which was unable to attack because of all those white tappy guys! Of course, I cast a Pyroclasm and won that game (with one life left).
—Ulrich

Another card I would like to mention is the Scarwood Hag from The Dark. I suggested this card to Ulrich after he told me about a particularly disconcerting game he had played where he had been forced to destroy his own Erhnam Djinn to keep his opponent from forestwalking him to death. The Scarwood Hag has the ability for four green mana and tapping it to give a creature forestwalk (a rather prohibitively expensive way to give forestwalk) or, and I think this is often overlooked, for the mere cost of tapping it, a target creature loses forestwalk until the end of the turn. This ability makes the Erhnam much less risky.
—Helva