May I just throw in a number of criteria that may be useful or not. I'm not very good at doing the maths in terms of karma=(a*b/17)^2+(e)*f, and I'm also not sure if i got the idea of what "developer karma" should be... anyway:

Port vs. native development
I once ported a command line application and used it on my device without publishing it. When packaging and extras became an issue (developers told us here that the whole process is broken and oh so difficult), I tried if I could bring it to extras without prior knowledge of debian packaging. I could. An I went the whole way and included it in /downloads/OS2008/. When I asked on the mailing list who I should name as the author of this package, I basically got "whoever you want to get karma for it". Now I wanted to be named as a contact person, I wanted receive karma,... but still: Is it OK to apply the very same developer-karma calculations for someone who'd only re-package an existing tool without even touching the source code? Maybe using two fields in the downloads page ("author" vs. "contact") could be a starting point to change this.

Dependencies
What about packages that are listed as dependencies of other packages? Say I port either a library (or an interpreter for a new language...) to Maemo, and 359 applications start using this library (or interpreter). Most probably nobody would ever download it from /downloads/OS2008/ or comment on it, but without it, 359 high-rated apps wouldn't have been possible. So: Count dependencies in packages.

Rating vs. downloads vs. comments
Relevance is an underdefined term. Whats relevant? Is a game relevant because it has high download rates? Is mplayer relevant because of its download rates... or is it relevant because it pushes the boundaries of the platform? I don't know. - I do believe, though, that counting only the average rating or the number of downloads or the number of comments given for this application will probably give a wrong result.
Downloads can be wrong for a number of reasons already discussed in this thread
Rating is a most irrelevant figure as long as it's the average rating. I give my own application 5 stars and have a higher multiplier than a really popular app that users find minor bugs in and that therefore ends up with 4 stars.
Number of comments could be nice to throw in. How many people think your application is worth writing about? (These could be, of course, all bad reviews like "What's that? It doesn't even install!", so it needs to be just one factor of many)

The penguinbait-effect
There must be a way to gain karma by doing things that don't quite fit the "install a deb from extras" scheme. Many interesting concepts that show how versatile the whole ecosystem is will not, cannot or should not be deployed to end users this way. Still, I'd personally consider them at least as (if not more) relevant than a game that gets downloaded 2000 times a week.
We used to have thumbs up/down on the maemo.org profile pages. This never worked for karma, IIRC, and is now disabled. How about adding a way for developers to add specific projects to their profiles (name of project, link to project homepage, short description, no screenshot) and allow others to give karma via thumbs up?

Fire exit
Karma calculations are one thing and will always be "wrong" for some people, no matter how good the system actually is.
The developer device program is another issue. It can be, but needn't be exclusively tied to karma. You could, say, make it public that a list of x developers will receive a discount based on their karma. Afterwards, those who're not part of this list should be given a chance to be elected by popular outrage ... you know, any developer should be able to stand up and say "hey! community! i think i do deserve it for my projects, but the karma system doesn't work for me. you know me. you love my projects. vote for me at...". we're missing the "at ..." for such a community-ticket.

non-developers
During the N810 device program, I read:
Is this still valid or will we fall back to developers only again? Just confused because this thread is about developers only. (And no, I'm not asking for myself... I live in a country without a Nokia online store, so I wouldn't be able to use any discount code, anyway.)