Currently the developers receive karma basically based in how many Garage projects are maintaining or involved with. This is actually a not very relevant statistic: one developer might have only one project making happy to 100.000 users (see Mplayer) while other might have opened 14 garage projects for 14 command-line direct ports of Debian that actually nobody uses or could care less about.
This is not easy to address as there are many possibilities to be unfair e.g. several developers in a project with different degree of involvement, apps downloaded by thousands that are "easy" ports of projects developed hardly by someone else. I wouldn't make big fuzz of this karma for developers, but it would be good to take it into account somehow.
Currently it's easier to get plenty of karma blogging about apps, talking about apps and commenting bugs in apps... but at the end are the dvelopers who carry with a lot of the hard work. Let's praise them!
Discussion thread: http://talk.maemo.org/showthread.php?t=31519