There are plenty of anime out there that felt like they were cut short. Maybe the plot hadn’t fully emerged, or you were just starting to care about the characters before it ended. Maybe the world needed more exploring, or episodes started getting rushed thanks to the time limit. Whatever the cause, we all know certain anime should have been longer.
I am purposefully leaving off anime that has a source material with the exception of #2. That’s because if they only animated a small part of the series, it means they were only doing it to advertise the manga anyway, which you should go read. Therefor, Berserk, Gunsmith Cats, Battle Angel Alita, Claymore, etc. will not be appearing on the list.