I think the biggest problem with any kind of system or government is corruption and dishonesty. Politicians lie to get their seats. The promise sunshine and rainbows, but once they get the power don't do anything close to their promises.
I think it is not too much about what type of system we choose but rather what people are running the system. That is our main problem basically in every country around the world. We can have the greatest system develope in theory but if the right people are not behind it it will not work.
At least that is my humble opinion about it. :)
Definitely, corruption is the problem, and any governing system must have checks in place to disrupt this from happening. Some of the suggestions in my post to part way to strengthening the defences, especially those that disrupt the idea of a politics as a viable career path.