Shouldn't POI's be points instead of complex polygons, just from a database perspective? Complex polygons are OK for true landmarks, which, by definition, are supposed to be few and far between (i.e., you really should not see 6 different landmarks on a single block of road; most of those things would probably not be actual landmarks then). There's no limit, however, to how many POI's you could have in any given area though.
A landmark is, by definition, prominent. I think one of the things that has to be decided is whether we want to limit landmarks to physical prominence - in other words, does the thing, say, visually stand out from the vast majority of things around it, perhaps for at least a few miles? Or, do we also want to include objects of social prominence as well - like, for example, does everyone - at least in the local area - know where and what the thing is even though it may not stand out visually like a tall building or some other tall structure? This would potentially include businesses with well-known branding along the side of the road. The caveat here, I think, would be that social prominence is far more subjective than physical prominence, making the system more prone to clutter, but we should probably also not forget that the social aspects of Waze are one of its biggest features that make it stand out from all the other cell-phone navigation options out there.