SynCaine is right: Guild Wars 2 does not scale perfectly when you visit lower-level zones. As you might expect, a level 30-something scaled down to level 4 is still more powerful than a level 4, and then the content was balanced around level 2-3. I confirmed this when I took a low-30s elementalist into Queensdale and one-shotted elementals with Arcane Blast. Fast-leveling guildmates also report feeling much more powerful in WvW as a 50 scaled up to 80 than as a 10 (also scaled up to 80); this reminds me of WAR scenarios, where you knew you were going to lose when half your team queued up a the minimum level. Scaling does not overcome the fact that higher level characters have more and better gear, traits, and abilities.
Some of that may be a factor of the small numbers involved. This is not hard math: 2 is half of 4 but only 10% of 20, so a two-level difference means a lot more at level 4 than 20. Visiting areas in the teens and 20s, the difference seems less severe, although I have not tested it vigorously. I also expect there to be some scaling tweaks in place by the time we have enough level-capped characters to test scaling over its full range.
As usual, I would like to point out that City of Heroes solved this problem years ago. Your entire group is set to the same level. You can set the content to whatever level and group size you like. Here you would scale the characters rather than the content, but the principle is the same. City of Heroes faces the same problem, that a level 50’s enhancements make him/her far more effective when scaled down to 20 than a level 20 character is, but you can always choose to adjust your challenge level if you want a more authentic level 20 experience.