There were so many..... The shows in which I experienced the crash and burn effect after a couple initial episodes:
Only Side by Side with You (was hooked till about 10 episode, now I put it on hold; may or may not pick it up again)
The Great Seducer (loved the first 6 episodes, but I'm getting a very different vibe now)
W (still liked it, but there is a sharp decline in quality after the first 4 episodes)
Protect the Boss (just couldn't handle it after the first couple of episodes)
Love Me if you Dare (still liked it, but the second half doesn't even come close to the first)
Goblin (huge mess when they introduce the secondary leads)
Me too, Flower! (from a really decent story to a bunch of pathetic characters making ridiculous life choices)
Across the Ocean to See You
And, the biggest disappointment of all times, from pure love (episodes 1-3) to pure hate:
Angel Eyes