It's easier to tell if the project will help your external career. Will you get to put some of the work into open source software? Will you be able to present it at conferences? Is it going to make an interesting line on your CV?
For internal career prediction, that's harder. My advice: just talk to people. The reputation of the project is a better indicator than any technical details. A project that is technically excellent but poorly sold internally is still going to be an effective failure.
Not necessarily. GMail, Google Instant, Knowledge Graph, and every visual redesign Google's ever done were all very controversial internally, with a number of people predicting that they'd be the death of the company. (When Instant was under construction, we had a betting pool going on in my team about how long until it'd be rolled back, something I'm glad to say nobody won.) All of those turned out to be quite large career boosts to people who worked on them, and in GMail & Instant's case, good career boosts externally as well.
I would evaluate external career boosts the same way I evaluate startup ideas. (Evan Williams has a good framework here:)
For internal career boosts, I would do the same, but eliminate the question about discoverability and monetizability, and view "wideness" and "deepness" through the lense of the power of individual executives within the company. It's not enough to hold a wide appeal to potential users, you need to hold wide appeal to the users that your VP cares about pleasing, or you need to deliver value along the lines of your VP's thought process.
For internal career prediction, that's harder. My advice: just talk to people. The reputation of the project is a better indicator than any technical details. A project that is technically excellent but poorly sold internally is still going to be an effective failure.