Rate Card on Utility in Limited - combination of raw power and efficiency from 1-5
Examples from recent sets:
5 - Pack Rat, Elspeth, Sun's Champion - Stone cold bomb,
4 - Sea God's Revenge, Agent of Fates - Very hard to beat
3 - Lash of the Whip, Spearpoint Oread - Solid picks in color
2 - Nylea's Disciple, Crackling Triton - Basic Playables
1 - Spark Jolt, Asphodel Wanderer - Doesn't do a lot, but better than an extra land.
0 - Pyxis of Pandemonium - Useless.

-AND-

Rate Card on playability in Limited - essentially how much of your mana base needs to be devoted to casting it and whether or not it goes in all decks or not.
5 - Late game, 1 mana symbol Sea Gods' Revenge (<4 sources) - premium utility spells
4 - Mid game, 1 mana symbol [c]Lighting Strike[c] (4-5 sources) - most removal
3 - Early game, 1 mana symbol, or expensive 2 mana symbols (not a splash) (6-7 sources) - most creatures
2 - mixed medium CMC cards (8-9 sources) - combat tricks or enchantments
1 - costs like or on curve- 10+ sources - cards of limited scope
0 - Triple mana or double-double - As many sources as you can squeeze in. - extremely narrow cards

In some sense you can think of "playability" as the converse of synergy. Cards that synergize with LOTS of cards or don't synergize at all are highly playable in any deck. If they are also powerful, they will get highly picked. Cards that really demand a strategy or 'build around' may be picked lower than their raw power level would suggest.

I leave the 0.5 values for the gallery to debate about