The problem with a strong president, especially in a country that doesn’t have a stable tradition of political parties or peaceful transfers of power, is that there can only be one. The whole country is plunged into a winner-take-all political contest. If the winner turns out to be someone with a “one man, one vote, one time” concept of democracy, then that’s all she wrote.
I add the proviso that you want either some kind of proportional representation (with a cutoff at 5% or 10% not 20%) or, shudder, a senate.
The English are quite proud of the unwritten constitution (writing it down is sooooo nouveau riche). However, they admit that it pretty much amounts to an electoral dictatorship.
If you have a disciplined party with an absolute majority in the one real houes of Parliament, then you are most of the way there to a dictatorship.
I live in Italy. Silvio Berlusconi avoids conviction for his many crimes, because he can change the law so that his acts, which were crimes at the time, are no longer crimes. Makes me long for the filibuster. Well not really, but Italy doesn't effectively have the rule of law (it is really irritating when a lawyer makes an argument to a judge that the law should be interpreted one way, the judge says know then the lawyer puts on his other hat as chairman of the justice committee of the lower house and writes a law which says, in effect, the old law is to be interpreted the way I said and is not to be intrepreted the way the judge intepreted it -- I am describing actual events). This would not be possible in the USA.
In constrast if you have a unicameral congress with many parties, the President has to negotiate.
Looking at non dictatorships, Jamaica was in a bit of a spot both when the Jamaica Labour Party (obviously pro-market right) won all but seven seats in one election and then all of all of them the next because the PUP boycotted the election.
In contrast Lula da Silva has been in an Obamalike position of being no threat to anything (not that he would be anyway) because the Workers party has never had a working majority in parliament. So he has to deal. Which means deal with crooks. Which means pay bribes.
Egypt is not currently a failed Democracy. They never tried. It was a dictatorship with window dressing.
Interestingly Democratic forces in Brazil and S Korea insisted very hard for a Presidential system. A gradual transition from military rule makes it very easy for members of the show parliament who are friendly with the generals to win, because they have been bringing home the bacon.
Finally I might add that the terrible power of Dmitri Medvedev isn't the worst problem with Russian Democracy. That winner sure didn't take all. To me, Putin seems just as much a dictator as prime minister as he did as president.