In the simplest of terms, the Wii U is entirely superior to the Wii. Though the name of Nintendo's newest console might suggest it's just an upgrade, this statement had no validity at all. The Wii U is an
entirely new console with new hardware, new functionality (obviously), etc.
As one should expect, Nintendo is only going to be developing high-profile games on the Wii U from now on. Judging from your signature and avatar, I'd imagine you're just a
tad bit of a Mario fan, no?

You won't be gettin' the newest Mario installments without a Wii U. In other words, the exclusive games. If you want the best and newest games Nintendo will be releasing, that should be a bit of motivation to invest in a Wii U.
In terms of pricing, I definitely believe that $300 is worth it. A Wii U's capabilities in terms of graphics and the like are comparable to the PS3 and 360, only superior of course. Both consoles were released half a decade ago still cost around $300 new. The Wii U is the newest release, has more horsepower so to speak, and costs the same, if not
less than these other systems. At the very least, the fact that on-release it's the same cost as competing consoles released years ago shows that the price is most definitely good.
A fact I was happy to learn upon the Wii U's price reveal is that the Wii U only costs $50 more than the Wii's original price of $250. The console's technology is leaps and bounds better than the ol' Wii's, so the fact the price barely went up certainly had me happy. If you're not ready to invest in the Wii U, the price will drop to even less eventually to the point it's a steal.
The Wii U's internet-related capabilities will be incredibly superior to the Wii's. It'l all be more user friendly, quicker, better polished, all that jazz. Online multiplayer games were known to have some issues on the Wii, but the Wii U promises to fix these up and then some. While this has yet to be seen in action since the console is unreleased, news so far points to the claims to be true.
Lastly, the Wii U will be fully compatible with any and all old Wii accessories, games, etc. You can sell off your Wii and invest that money towards a Wii U without fear of havin' a library of unplayable games and useless accessories. In fact, some new games'll use both the Wii U's new controller and the Wii Remote. Thanks to this feature, you won't need to buy an extra Wii U controller(s) for local multiplayer. Nintendo doesn't actually sell individual Wii U controller for this very (cost efficient) reason.
Though the console itself isn't groundbreaking-epic in terms of the best graphics in the world or new technology, it's better than the Wii in
every way. If you were happy with the Wii, it's nigh-guaranteed you'll be doubly so with a Wii U. The price fits the technology you get (in my opinion), and more importantly, Nintendo has always been known to reliably sell games of the highest caliber. Even if the console was a tad overpriced or a bit underwhelming, I'd still buy it knowing some of my favorite games will be 100% exclusive to said console.