It's not high base damage so much as the sheer power of having four nuke spells. Four damage spells that deal damage upfront (mostly), they aren't damage over time or whatever. This is also what makes Brand powerful. Few mages have four damage spells. Most have three (Ryze, Lux, Annie, Veigar, Xerath), or even just two (Sion).

Land all four of LeBlanc's spells - Sigil, trigger it, Mimic, trigger it, Distortion, Chains - without missing your chain, or wasting distort to get into position, and that's a huge amount of raw damage. This is what gives LB her feared early game.

Once people start buying health, you're going to need AP if you want to keep up, as mages do not scale against max health. People talk about LB's bad late-game, but her scaling is deceptively good, as two of her spells are broken in half. Sigil actually has a .9 AP ratio all together, and Chains is a full 1 AP ratio if both parts trigger (.5 on each part).

The power of DFG is that it adds a fifth spell to your burst rotation. More damage is always good.

I'm confused. why would you prioritize spell vamp on a frontload mega burst caster?

Great insight! Thank you

It's bad advice. You don't.

I tend to build Leblanc a lot more focused on her utility, IE movement, because there are two types of people you'll be fighting, those you can kill and those you can't. Now Leblanc can't burst a bruiser or a tank EVER regardless of your pen unless you are seriously fed and they are stupid. However, Leblanc can kite a tank or a bruiser for a very long time if built correctly. I personally like to build 5 speed boots(I cop **** for this when I play with 1400s but have never had a 2000+ complain)(they also help you get around the map seriously faster) haunting guise or sweeper(depending on their team) then Rabadons and Morrellos depending on what I need at the time. You can still burst squishies(IE those with less than 60 mr) but you actually have a chance in a fight against 90% of peoples picks if you are smart about your kiting. This CDR build also is great for defending points because like Ashe you can pretty much kite forever.

Flat MPen + DFG, transitioning into Void + Deathcap late game if the entire enemy team stacks MR. You'll almost always have someone to instagib, though, and forcing the enemy carries to buy MR means you're doing your job anyway.

I was in a game with you, noticed your boots, and simply didn't feel like saying anything. You couldn't burst their Ashe down when all she had were merc treads for MR; you badly needed the spellpen from Sorcs. Mobility boots are unneeded on LB, Sorcs match her kit and role perfectly. Considering 3/5 of the enemy team itemized no or minimal MR, you would have done a lot better with that flat 20 Pen.

While you are correct that Leblanc lacks the.. consistency of champions such as Kassadin, Ryze, Fizz, Ahri, she has a much higher skill cap because of her versatile ult.
The characteristics of a strong caster champion on dominion are Mobility*Survivability and the aforementioned champions have an excess of one of these traits or a strong combination of both.
Leblanc is a bit lackluster in that she has excellent mobility but isn't very tanky and basically this is where player skill comes into play.

That was a tough match up for me because Ashe, Warwick, Pantheon, and Jax all had 80 magic resist before level 10. I like 5 speed for being able to actually support my team, I hate how slow I am to walk across the map with 2 speed. I mean sometimes I go with Sorcerer's Treads depending on the match up, but I've had a much more favorable response overall with Mobility. I tend to think that forcing someone to B is the same as killing them, in that they no longer are able to support their team. I am much more team focused than kill focused compared to most Leblancs.

This being said there are many ways to build and play a champion and mine is probably not the best, but I do what I can to build a kit which complements my abilities.