The best example of good start / bad ending I can think of is the recent Jyu-Oh-Sei. The beginning is awesome, but the end is a long string of "wtf?"
Fullmetal Alchemist had a crappy ending too...
It's harder to think of bad start/good ending shows, though. I can't speak for the entire series, but I've seen half of Eureka 7 and thought the beginning was crap, but after giving it a few episodes to warm up it turned out to be wonderful. I guess bad start/good ending shows are so hard to think of because if the first part of an anime is horrible, people won't watch it to the end to find out if it's good.
