I've already said something, but it wouldn't hurt to add a little bit more.
Generally, I find shorter series to be forgettable, even the really good ones. When I watch them, I'll enjoy them immensely, but after a week, they're all but gone from my head. I remember them, but I never think about them. After a year, I may not even be sure what the series was about.
This doesn't happen with long series, however, because it takes a long time to read/watch them, especially if the series is still going on. Also, they tend to be grander, with more things that will stick in your head. A 12-episode series cannot have a gigantic battle that lasts for six episodes.
For example, I watched the Sketchbook anime a year ago or so, and I loved it. I still remember bits and pieces of it, but I don't know any of the characters' names or any of what happened in the series. On the other hand, I have been watching/reading Naruto, Bleach, and One Piece for years. Every week, I look forward to a new chapter. Also, I am so hooked to the series that I cannot wait to read each new chapter; on the other hand, I've stopped watching the short, currently-airing anime series because I can't stand waiting a week for each episode. Either they aren't interesting enough that I forget about them, or I don't remember who the characters are and what's happening. So, I wait until those short series finish, and then I check them out. I've thought about stopping reading Bleach, Naruto, or One Piece for a month or two, so I don't have to sit on pins and needles every week while waiting for the next chapter, but I'm never able to go through with it, because of my desire to see what happens next.
With long series, I can't wait. With short series, I can.