Unfortunately complaining about about downwind spacing and the size of other people's circuits is just like complaining about slow play in golf. It always happens and there isn't much you can do about it in an unregulated area.
The concept behind different circuit heights is to initially separate aircraft based upon performance and prevent a faster aircraft overtaking/descending into another aircraft. By the time they are at base turn and commencing final descent (remember - base and final is where most mid airs occur) hopefully the pilots have heard 1 or 2 radio calls from other aircraft and have positioned themselves appropriately.
For this to occur all pilots must be diligent in ensuring they have a good visual scan and using the radio (transmitting the standard radio broadcasts and LISTENING). Obviously this doesn't always occur, people do make mistakes, but other than education there is not a lot else you can do in a unregulated area. The next option is a third party to pass traffic information.