As a start, they have to be simple enough to use to allow new players to ease into the game At the same time, they also need to be interesting enough to ensure a good play with them and against them
Jecht, Reluctant Guardian Tidus dad here is a bit surprising to see in black, but has an acceptable front face, and his back face is highly disruptive to opponents while drawing you cards and giving you one attack with a 7/7 Menace
It also works well in decks that have beneficial death triggers I could easily see it as a good piece for a Jund sacrifice deck
For example, the sets Collector Boosters a product normally priced at $27 or so are listed for a frankly unbelievable sum of $37.99
Reference Deck Overloaded (ECL) Rare/Mythic Rare Ajani, Nacatl Pariah20 Rise, the Ever-Changing40 Slumbering Sentry40 Common/Uncommon Unfair Strike(Unc)100 Lockout(Common)100 Quick Note A deck featuring -1/-1 counters and convoke