If you still going by "a simple layout and design." I think the extra extensions are making the punbb forum look unprofessional.
Karma? Who needs karma on a software site, phpbb, SMF and all sorts don't have it... Yes positive karma could help if someone does a good bit of support and stuff but negative karma? you don't want that on a friendly community site! And if you knew who gave you negative karma you would do is negative-karma that user back resulting in cyclical karma giving.
If you install loads of extensions then your have to put them in the punbb package otherwise it gives the impression they are default features, then again your moving away from "simple layout and design"
Then the BB codes on the forum, you don't need all of them for a simple forum, personally you don't need the '...' ones.
