Building your capabilities is the key to getting high scores and more enjoyment from this game. As you gather coins, investing them in these various ability boosts is essential. The first one or two purchases per category are a bargain but as you can see, the cost of the more advanced abilities increases dramatically.
My favorite abilities, as you can see below, is increasing coin value, boost distance, pickup spawn and power meter. Score multiplier is also important.
Definitions of each Ability:
Coin Value: Sets how far you have to run before you start seeing the blue coins which are worth triple points.
Shield Duration: Sets how long your shield will last once activated. The more you advance this ability, the longer the shield will last. Shield allows you to avoid tripping over hazards (logs, bridges, streams...) but doesn't save you from missing a turn or dropping into a chasm. It also lasts only once - after it protects you from a trip, it is gone.
Coin Magnet: Sets how long the coin magnet will last once activated. The more you advance this
ability, the longer your coin magnet will last.
Boost Distance: Sets how long the fast-forward boost distance will last once activated.
Pickup Spawn: Increases the frequency that boosts will appear during your run.
Power Meter: Increases the speed that your power meter fills up as you gather coins. Faster fill up = quicker time to get to your boost.
Save Me: Pay less gems to resurrect yourself after you die. Critical to keeping that long run going even longer!
Head Start: You can buy a Head-Start for less and less coins as you build up this ability (which ironically costs more and more coins). It gives you about a 1000m head start
Score Multiplier: Increases the rate at which you score as you run. Thanks to a commenter, here is additional definition: For every meter you run, you get a number of points equal to your Score Multiplier.
For every coin you collect, you get a number of points equal to your Score Multiplier x5.
Every time you use the Bonus Score powerup, you get a number of points equal to your Score Multiplier x1600.
None of the above values account for improving powerups with gems