Arkose: It's not the specs (Tegra 4 is definitely capable), it's the games. Your Skyrims and Call of Duties and whatnot largely don't exist for Android except in very simplified forms built primarily (or exclusively) for touch input and centred around short-term play. Proper games won't become more common until portable/console-style Android devices gain sufficient popularity, but such devices will be a hard sell without suitable games to run on them.
NVIDIA is well known for working with developers, even paying developers, to produce products which take advantage of its hardware. If the Tegra 4 is capable it won't surprise me if NVIDIA will work with companies producing console titles (especially its TWIMTBP partners) to convert them to work on the Shield (and other Tegra 4 devices).
As for success, it remains to be seen, but NVIDIA certainly hit a lot of nails on the head with Shield, so there's a chance it will succeed even without a big initial rollout. Still, I won't be surprised if NVIDIA has up its sleeve a launch lineup or at least a game or two showing the full power of the Tegra 4.