The level 18-20 roadbump is a prime example. The xp in that range is on the low side. Most players agree on this. The player solution is to stay as low a level as possible to increase your options for quests to run for maximum xp. This often includes RWtD, Rainbow, Kobold, etc. The "big" xp quests encourage you to stay level 18 max for the best xp/min rewards. I know it's been mentioned time and again that you don't "have to" do this to level up, but players flock to the path of least resistance and that path does not include Amrath or Cannith quests currently. Raise the xp in those quests to be competitive xp/min and suddenly banking levels 19 and 20 becomes less of an incentive.
The issue is similar in the epic levels. Unfortunately, while heroic levels offer your character advancement through enhancement and class levels, epic offers none of this. Most of your power comes from your destiny, which is independent of your level xp. 90% of your character advancement from xp comes from your destiny. It was a poor design that is coming back and biting Turbine now. Your carrot for leveling your character epically is your gear. Right now, you want level 23 because you may have some stashed CitW weapons for your next life. You might want level 25 so you can slip into your dragon scale armor. When the new pack comes out, I want level 28 because?... Well, I don't have the gear yet, so that's not really an incentive. The gear incentive only really factors when you have the gear banked from a previous life or possibly passed from another toon (in the case of GH gear that is BtCoE). The only reason for a character to level up, besides a few hp and a feat at level 24 is to get to their gear stops. These gear stops don't exist until you have the gear.
Overlevel xp penalties are a massive incentive to NOT level up and this is something that Turbine really needs to address as it's the core reason why players bank levels and is a disincentive to leveling up. Players flock to the best xp/min quests and often that leads to incentive to not level up because of massive droughts in xp at certain level ranges (quest level 10, 15, 19, 20 and 25 currently in my opinion). The level 10 xp gap is masked by the amazing rewards in VoN 3 and Shadow Crypt. If those quests are "rebalanced" expect this level range to hurt quite a lot more than it does now. Level 15 was helped only slightly by the newest pack as the xp is average at best and the initial elite run is a slog though nonthreatening mobs who are simply sacks of over inflated hp. If Litany is also "rebalanced" then this level range suffers massively as well. There are, simply put, only two actual level 20 quests. One is a raid and one is commonly run only on casual for the rewards. Level 25, the current cap offers nothing for 6 man quests. There are no base level 25 quests at all. The expansion may only be adding 3 levels, but the cap of the previous expansion still needs to be addressed. Adding 10 quests and expecting them to fill 4 levels worth of xp is just an unreasonable expectation.