Since this is "disappointing or boring" and not "outright crap" I suppose I can avoid mentioning the obvious ones of Madoka, Fate/Zero, Attack on Titan, Sword Art Online, etc...
Akagami no Shirayukihime. God, this show is so boring, I can't believe it warranted two seasons. It had an interesting premise and a vaguely interesting beginning, but all potential quickly evaporated when it devolved into a stereotypical "boy meets girl, boy likes girl, girl likes boy" sort of thing.
After an explosive start and breakneck plot, Dokidoki Precure ended up being incredibly disappointing after the mid-season finale, too. Half the plot intrigue was centered around Regina, and they took her out, brainwashed her, and tried to replace her with Cure Ace, who was an awful character. Watching Dokidoki week-by-week, I loved it...right up to that point.
Yu-Gi-Oh! GX and 5Ds both became this for me at points, too...with GX it was right around season three - I'm sorry, but the Supreme King plot was just BAD - and with 5Ds it was right after Aki lost her personality and turned into a schoolgirl. Both of them were extremely disappointing shows and I never even finished 5Ds because of it.
The less said about Gundam: Reconguista in G, the better. This turned out to be extremely boring; I can't believe I forced myself to sit through it all. The ugly as sin Eureka Seven art style (speaking of, add Eureka Seven to the "disappointing" crowd too) didn't help matters either.
There are others. I'd say a good 90% of the shows I've seen based on ongoing light novels or manga fit the disappointing category because they cut off just as things are starting to get good, and it ruins the whole experience. If it isn't finished, don't adapt it, for crying out loud.