There isn't one.

Well, sort of.

Here we go. The reasons why there is not a significant gap between the elite rookie Genin main characters and the Jounins we've seen.

Sasuke:

There are several power showing for Sasuke that put him at a very high chakra level. First off, he freakin' tooled on six Mizubunshin clones of Zabuza (1/10th Jounin power each) effortlessly. This is the beginning of the series and already after the brief tree-training (a FAR smaller training period than the later one he undergoes in the Chuunin Arc) he is able to effortlessly smoke six clones. Eariler, ONE had been stomping him. In a week's worth of training Sasuke's chakra level got multiple times higher, putting him at a solid 1/5th Zabuza powerlevel, IMO.

Then, of course, we have the Haku fight. Haku is Jounin level - this is inarguable. Sasuke was defeating his Bloodline Limit with the Sharingan - slowly, of course, and not in a way that would be useful in a fight. But, again, this is Sasuke in the second-weakest portrayal he'll have in the series, and he's learning to defeat Jounin level techniques. In a few minutes' time, Sasuke went from being totally owned by Haku's Bloodline Limit to the point where his insight was catching up to Haku, a kid who's got skills and power that surpass Jounin level.

Sasuke tricked Orochimaru in battle. Sure, his techniques were pitifully ineffective, but Oro wasn't faking the look of surprise when his ass exploded. So we have cleverness that at least made an EFFORT against a Sannin. Yes, Oro wasn't trying. Point is, he WAS going to kill Sasuke if he found him to be unsuitable, and Sasuke proved himself worthy. This is a decent showing.

Sasuke smokes Yoroi in one combo after being chakra drained for several minutes. Above average Genin level? You bet.

Sasuke is shown to have half the Chakra of Kakashi when he isn't using the Seal (during the flashbacks to the training where he learned Chidori). With the Seal, he was able to muster 75% of Kakashi's level (3 Chidoris in one day). That he mastered a Jounin level technique is also a nice feat.

Sasuke smoked a non-transformed Gaara. Badly. He also harmed the partially transformed Gaara. This, IMO, puts him at Jounin level, easily. I'll do Gaara in a bit.

Naruto:

I think it's pretty obvious why Naruto's Jounin level: he has the fucking Kyuubi stuck in him. Whenever he uses it he trashes Jounin level beings. He tosses Haku, BADLY. Haku is Jounin level. He uses a freakin' Hokage level Kuchiyose (or however you spell it). He broke through SHUKAKU's defenses to attack the sleeping Gaara. There's another piece of evidence I'd add here, but that's a spoiler. Anyway, yeah. Naruto has waaay above Jounin level chakra.

Sakura:

Just kidding. She's weak as hell.

Rock Lee:

Tossed the non-transformed Gaara around like a ragdoll. Yes, he lost. Losses do not mean that the opponent 100% outclasses you. In fact, Gaara barely outclasses Lee at all.

He can open the Celestial Gates. Again, don't feel the need to explain why an ability that moves you to Hokage level once you master it is a Jounin level technique when partially accessed.

Gaara:

Obvious, again. He has a fuckin' demon in his body that he draws power from and he's the child of a Kage.

I think, though I am not sure, that Yashamaru was a Hidden Sand Jounin. Gaara murdered her/him effortlessly - as a child.

Baki fears Gaara. Indirectly, we can assume that Kazekage feared Gaara as well.

Jounins:

The position of Jounin has never been linked to having a huge amount of Chakra or a huge amount of ninja fighting skill. In fact, the OPPOSITE associations have been made: I'd elaborate but I don't believe the results of the Chuunin Exam have been released in the anime, have they?

I'll say this: high rank ninjas are not elected for their fighting prowess. So why is there this huge assumption that random Jounin #47 would whoop a bunch of Genin's asses? I feel that pretty much any Jounin who isn't an elite level like Kakashi would get defeated in a fight by heavy hitters like Sasuke, Gaara, Neji, Lee, or Naruto. Again, I'd add more examples but they're spoilers. Hell, I'd give some of them even odds or better against Kakashi-sensei.

So, yeah. I spent a bit too much time on this.

What do you think? Coherent? Logical? Factually based?