One other point about “first past the post” compared to proportional representation in a parliamentary system: it has been argued that a proportional representation system contibutes to weak government and even corruption.
The Italian example is often used. Until constitutional reforms in the late 80’s, the average length of an Italian ministry was less than one year. No party ever had a majority, so there was constant brokering and deal-making. As well, one party, the Christian Democrats, was in the Cabinet in almost every goverment from WWII onwards. As a result, it got very comfortable in government, and corruption ensued. No matter how much the people may have wanted them out, the Christian Democrats stayed in government.
By contrast, the virtue of the “first past the post” system is that it allows the current government to be completely kicked out of office, and entirely new, fresh people to come in. It has a purifying effect both on policies and corruption. You have entirely new ideas brought in, and the risk of corruption reduced, since corruption tends to develop over time. (I’ve seen it referred to in Britain as “removal van democracy,” referring to the fact that the defeated Prime Minister is expected to be out of 10 Downing Street within 24 to 36 hours.)
Now, there are a variety of proportional representation systms, such as the “mixed” system used in Germany (some candidates elected directly, some from a list), which may produce different effects. But, I think the electoral system has to be considered in a discussion of term limits.