I recently did the "light level climb" after returning to Destiny for The Taken King. I'll give you my rundown on what to keep and what to shard:
Keep whatever has the highest light level in each slot. (duh ;) Make sure you equip your highest light level gear when decrypting, since that will impact the level at which the engrams decrypt at.
Shard almost everything below the highest light level in a slot, if that item's light is less than 30 below the light level cap (so 290 at this point). So if you've got a 290 helmet and pick up a 260, shard that 260 mo'fo.
One specific exception is the purple sword you can get from one of the questlines - infusing this up to 280 is a hidden quest step. (Thanks, Ob- ...ungie) You can re-buy the swords now, though, so destroying your purple sword is more of an annoyance than anything else.
There is one weapon perk (Relentless Tracker) that temporarily grants you the ability to see chests and upgrade materials after getting a kill, and if you can find this, it's worth hanging on to if you don't play a Hunter and have this as a sublcass perk. (You'll thank me later when you're collecting Hadmium Flakes for hours on end...)
"But, what about this awesome 210 whatever?!?" - Almost everything can be re-bought in the tower once you meet certain prerequisites. For instance, if you shard an exotic, you can buy it back (albeit at lower light level) from the vending machines in the Vanguard hall. Shaders, emblems, sparrows, etc, can be bought from other machines in the Tower as well. The faction reps in the tower sell various purple-grade gear too.
Another consideration - sharding purple items grants Legendary Marks. There's a cap of 200 marks you can hold at a time, so if you're at 200, you might want to save purples to shard later when you need more marks. Of course, you could spend some marks beforehand if that suits you.
If you are using Exotics, bear in mind that you can only have one Exotic weapon and one Exotic armor piece equipped at a time (again, some special exceptions apply, but this is the general rule). Thus, it makes sense to save a non-exotic piece with good light for slots where you might choose an exotic, so that you can use different exotics and not hinder your light level overly much.
When it comes to stuff that's within 20 or 30 light of the cap (ie, 290+ gear right now when the cap is 320), things get a bit trickier. You may want to keep some of this stuff so that you can infuse it into other gear. For instance, many exotics drop at 290, or 280 if you buy from vendor(s). Thus, you might want to infuse these up to get the most out of them. Or, you might get a really good legendary that you want to level up, so having some sacrificial high light items can be worth the trouble of storing for later.
(Infusion keeps a percentage of the difference in light level unless the two items are close enough together to get the full difference. If this is confusing, consult an infusion calculator ;)
A final note - standard Crucible (NOT Iron Banner) matches don't take your light level into consideration. Thus, you might want to keep some weapons that suit your playstyle around (ie, year one exotics, or weapons with your favorite perks, etc) to play regular Crucible with.