1. worst idea: Not having a harddrive as standard like the PC. As games became bigger, they were required to be installed on the hard drive. Amiga's had superior games, thus a hard drive would have been even more suitable for Amiga than a PC. Games were not harddrive installable and it made Amiga look unprofessional. At the time, people using computers wanted their computers to look professional. For example, they wanted to use DOS commands to install their games to harddrives. Today it's the opposite.
2. worst idea: Using a Double Density floppy drive instead of a High Density drive like the PC. Games became bigger but Amiga's had no harddrives as standard and the double density drive made it even worse.
3. worst idea: AGA. It left something to be desired. The first thing I tried was to set the Workbench resolution to 256 colours and 640-512 screen resolution. As if the flickering was not enough, everything became extremely slow. They should have made it much faster, flicker free and it should have "killed" the A500 so everone started buying AGA Amigas.
4. worst idea: They should have made Workbench 3.1 with built-in functionalities for networking, Dopus Magellan 2, 256 colour icons and built-in fonts for different countries. Datatypes for jpg, wav, etc and later made mp3, avi, etc as downloadable updates.