I really miss something like that.. like filters per subscription or something.. how do you handle unwanted posts in feeds you actually want to read?

That sounds logical, but isn't. For one, unlike mail content, which Gmail can scan/index/classify in full well in advance of rendering, RSS-streams by default represent but fragments of ultimately-delivered full content, usually just a headline plus a short snippet of each item. Some blog/ web publishers push their entire content in RSS form (sometimes even in parallell, and quite granular streams devoted to inividual response threads), ie. provide RSS-aggregators such as GR with full, potentiall-indexable content, but they're in clear minority, as each so-"consumed" blogpost means one less traffic-/ revenue-raising visit to their own home sites. So built-in filtering could at best depend on the presence of very short item titles, far shorter even than mere 140-chars tweets.... not a good starting point.