Focus on planning, not leveling. To defeat the black dragon Zolanderos, the CR 10 terror of Staunwark Island, the heroes will need help, special resources, and information. I want to further encourage party-directed adventuring, and if the heroes want to take on something 4 to 6 CR above them, then that’s what they will require.
A low magic game that everyone knows how to play.
Never a need for meaningless encounters. The players can be involved in a dozen or so major combat scenarios (perhaps more than one encounter each) and have proven themselves and made a major accomplishment. See Lord of the Rings movies, or most fantasy novels.
Classic monsters stay classic throughout the campaign; Chimeras and Aboleths start scary, and stay scary. Dragons are always exciting encounters.
Even legendary heroes remain mortal; while a 6th level fighter who has taken toughness several times can take on a good mob, he isn’t invulnerable. The sorcerer’s 6d6 fireballs are phenomenal, but not so powerful that he can destroy a village and not fear retaliation.
Quicker prep. Make a 1st, 3rd, 5th, and 6th version of a sorcerer, and now you have a whole sorcerous dragon-cult that can last you through your whole campaign.
You can put what you’ve learned of the rules to good use. It’s hard to know every 4th through 9th level spell out there; they’re the ones we see the least. But we’ve seen 0th through 3rd level spells many, many times, and mastery over them is relatively simple.
E6 is a great system for on the fly GMing. If you’re reasonably familiar with what a 2nd level threat looks like, power-wise, you can probably get away with running it without stats handy.