I've gotten my warlock to level 27 through lots of crucible matches & many, many strikes with a few friends. This is where i've gotten into a leveling slump. So the way leveling works after level 20 is light, a new stat that armor starts coming with, grants you a new level after you're wearing armor with enough of it.. There are blue, purple (legendary), and gold (exotic) gear that have progressively larger potential light stats.
Each of these pieces of armor can be upgraded with glimmer ($), materials you find in the different planets, and "ascendant shards". Ascendant shards are hard to come by, either having to complete the daily heroic story (story level chosen daily with difficulty modifiers), completing the weekly heroic raid, completing a public event, or (from what i've read) a potential reward for Vault of Glass Raid. I've also heard if you buy legendary equpiment from the vanguard or crucible vendors and deconstruct them, there's a chance at gettin g ascendant shards. Lastly, I've read about several people having two alts at level 20+ for the sole purpose of farming these materials (I don't know how anyone has that much time).
The problem with this is that the possibility of getting either this or the ascendant materials to upgrade weapons is completely random, and there are only a certain number of things you can do on a daily or weekly basis to obtain these materials. Basically, if you don't have a pile of people to organize daily/weeklies/raids, you're gated pretty heavily when it comes to leveling up at a certain point.
This game is like a candy apple with dirt covered all over it. At its core, there's great gameplay, but there's this layer of crap that makes it an exercise in frustration sometimes.