First, let's differentiate vertical progression from horizontal progression. Vertical progression is increasing your level. Horizontal progression is increasing your options/customizing your build, while staying at or around the same level.
When TTK released, vertical progression was of paramount importance to unlocking new content. Once you approach 300 light, the only way to go higher is by raiding, as blue and purple engrams cannot decrypt at higher than 300. At this point, it’s time to start customizing your character’s armor to complement your playstyle, and to make the content you like to play, easier. Note: going above 300 light does not currently help you, as 300 light is the highest-leveled content in the game (Oryx and exotic sword strike). It will help when hard mode of King’s Fall is released, which is why horizontal progression is so important at this moment. Get the build you want, then use raid gear to vertically progress toward the absolute ceiling of 310.
For brevity’s sake, this guide will focus on PVE but still applicable on PVP.
Customizing your build comes down to armor perks. Perks can complement your weapon choices, and make content a hell of a lot easier AND more fun. Let’s look at some of the perk options available. (This will not be a comprehensive list. Trying to be short, and they’re all posted elsewhere.) Choose what you feel will benefit you the most.
Intellect/Discipline/Strength can be important, as well, but I feel as tho perks are more important. If you find two pieces with the same good perks that have different stat spreads, then by all means, customize for maximum cooldown. (Just my opinion, tho.)
Helmets: Helmets affect cooldowns, as well as what happens upon orb pickup. Go for “more super energy upon killing minions of the darkness”, and “shield recharge happens immediately upon orb pickup”. Some of these can save your life, and/or make you a more efficient killing machine.
Gauntlets: Gauntlets primarily affect reload speed. Use scout rifles exclusively in PvE, so anything besides faster scout rifle reload is junk to me. Sniper/shotgun/heavy weapon reload can be nice in a pinch, but go for gauntlets that increase the reload of your primary if you can, it’ll come in to play 10 times as much as the others. Increased grenade throw distance is a nice bonus perk as well, but not necessary for me.
Chest: Chests affect elemental armor, as well as how much primary OR special ammo you can carry. Try to get a chest that boosts armor for the class you use the most (get 3 chests if you use all 3, if possible). The increased armor isn’t a game-changer, but it helps. For the ammo boost, go for your most-used special ammo, unless you use a hand cannon primary. The others usually don’t run out, but +sniper ammo for the raid (as an example) comes in super handy.
Boots: Boots affect agility, as well as special and heavy ammo. The agility boost after a double kill hardly ever matters. As far as which ammo boost to go for, this is totally up to you. Either run extra machine gun or rocket ammo if you use it a lot, or stack a special ammo boost with your chestpiece. Shotgun ammo on one and sniper ammo on the other can come in handy, and require less armor switching. Duplicate ammo boosts on both chest and boots will stack. If you find yourself not using your heavy too much (or you use sword a lot), this might be your best option.
Class items and artifacts don’t affect too much, so just wear the highest light one you have, or whatever looks the coolest.
Again, everything applies to PvP as well. Once you have the pieces you want, start infusing them past 300 with raid gear, and you’ll be much better prepared for whatever Bungie puts us through next. Hope this helped some of you with customizing your characters. Any comments/feedback are more than welcome. :)
Note: Raid gear all has raid-specific perks in addition to the normal perks, so if you get a good piece of raid gear with the right ammo boosts, etc., hang on to it. Helmets get a chance for bonus precision damage on Taken after orb pickup, gauntlets get faster reload in the invincibility aura, chest pieces give bonus armor when carrying the spark, and boots give bonus agility when carrying the spark.