I'm very new so these comments are very general.
Expert rank should be earned. While stars are nice, perhaps you can have 5 stars and ranks of stars.
For example, I'll likely be a munching cop-reporting commuter who won't spend a lot of time in cartography, at least at first. If I never change from that style, I still should be able to see progression. That progression is what will keep me exploring.
Now, reading the forums here there are some pretty serious people doing some real heavy lifting. They should be rewarded as well.
Yellow Stars 1-5: Stuff a casual commuter can do with his thumb and a sense of adventure.
Green Stars 1-5: You can turn a star from yellow (or nothing) to green by doing the next most valuable activity (perhaps mapping new roads, confirming alerts, etc.)
Blue Stars 1-5: Highest/most valuable activities, the hard stuff that takes real work, dedication, and reputation. You can turn green stars into blue by doing this stuff, which probably would include real mapping, fixing maps, etc.
Star ranks should be progressively harder in each color, a geometric progression so that making 5 stars is hard to reach. First star should be reachable in a week or so, 5th should take real effort over the long term and constant dedication. A yellow 5-star should be proud and have to work to keep it by munching and reporting.
Benefits: it rewards casual particiaption over the long term, which is vital for the growth of the community and hence the quality of the maps. It also rewards those who put the extra effort in, and makes it easy for the rest of us to see who they are -- someone with 5 blue stars will have earned it by more than jsut being around for a long time. It may also encourage some of us casual types to try our hand and more heavy lifting.
Drawback: More confusing than just 5 stars.
Just my $0.02.
(Edit: another thought, because I can't sleep.)
Perhaps at higher ranks you open up special avatars. Let the people who earn them show them off on the Waze maps!