(This post inspired by Petteri Räty (betelgeuse)'s similar post

For this year's Gentoo GSoC projects, I'm a mentor on two of our suggested ideas (but also interested in totally new ideas that fit my fields):

• upstart on Gentoo
• Distfile Fetcher Intelligence

Do you actually understand the project idea?

This is actually a gap that I didn't expect to exist, but I have seen in previous years. This is mainly a difference of expectations between the proposal and what the potential student sees as what the idea really entails.
Using Upstart as an example, it supports an existing init.d compatibility mode, but we're not interested in that. Instead we want our init.d scripts to be treated just like upstart jobs (located in /etc/init/). The init.5 manpage shipped with upstart gives a good start...

Code maintainability

betelgeuse spoke about long-term maintenance, but you should think about it long ahead of that. Some degrees of abstraction, and avoiding difficult to understand logic should be prevalent here. betelgeuse mentioned spaghetti code, but it's important to realize that even well formatted code can impose a much larger mental workload if not well thought out.

Timezones, Timezones!

Most of your project should not be blocking on asking for mentor advice, as timezones and real world pressures often conspire to prevent easy real world communication. I may live in UTC-7, but my hours drift as needed by work but I tend to be online anywhere between 17h00 UTC and 10h00 UTC. If you're trying to communicate with me on a regular basis, this can be tough, so being able work on a problem independently, ask highly directed questions via email can go a long way.