Anime that are not based on manga tend to run for one or 2 sets of 13 episodes (13 episodes is one 'season' methinks), but they can run longer. Things that aren't based off an already existing product don't have anything that qualifies as 'filler'. Like Eureka 7, it's 50 episodes of original work (not based off manga). There are some slow parts that people might say are 'filler', but it's all part of the overall story. There is an Eureka 7 manga, but it's based off of the show.
Jump! anime, however (Naruto, DBZ, Kenshin, Bleach, etc), are much more prone to filler--things made to fill in the gaps between the official/real story of the original work.

Some anime certainly have more material to continue, but they end in a good place (like Cowboy Bebop). Besides, there are always movies and OVAs to be made. They can keep their new and good ideas condensed to a 90 minute movie or a short series of episodes instead of having to 'fill' up a 26 episode season with degenerate quality.