As for the debate about parliamentary versus presidential systems, my personal view is that I prefer the parliamentary system, because it avoids the risk of a deadlock (mentioned already by others) between the executive and the legislative branch if the two are mutually hostile. Now in this regard, there seems to be a bit of American exceptionalism: The potential for this deadlock in the American system is massive, but by and large it doesn’t happen. The Senate, for instance, could seriously sabotage the work of the executive by rejecting all or most presidential nominees that require Senate approval (which, mind you, applies not only to judges but to lots of other appointments as well - for instance all cabinet secretaries and ambassadors). This does not occur; there seems to be an unwritten consensus in American politics that even in a very polarised time, the Senate doesn’t reject presidential appointees for purely partisan reasons unless there is a stron reason in a particular candidate that makes that candidate appear unqualified. It’s unwritten conventions like this that make the system work, much more importantly than the black-letter text in the constitution. And that is why you could copy the text of the constitution and other statutes verbatim to other countries but still get a very different outcome, if these unwritten conventions are not there. Most of Latin America has copied the U.S. model quite faithfully, but not managed to get it to provide even remotely the same degree of stability. So to sum this up, I think the presidential model has worked well for the U.S., but the parliamentary system is, in case of doubt, the safer bet. The French-style semi-presidential system is the worst, in my view; it it blurs responsibilities and leaves it unclear who is, actually, in charge - the independently elected president or the parliamentarily responsibile prime minister.
As for the structure of the parliament itself, I can see the point of bicameralism in a federal system where one chamber represents the individual components of the federation, but in a unitary system I would be strongly in favour of monocameralism. There really is no point in having two chambers represent the same electorate. The British House of Lords works only because its powers have been reduced to a largely ceremonial role; and the fact that almost all individual states in the U.S. have, at state level, both a senate and a house of representatives is, in my view, an oddity that comes from a desire to mimic the federal level, rather than provide anything meaningful in substance.
As for entrenchment clauses, I’m skeptical about their usefulness. They can work in a system with a strong tradition of judicial review and of respect among elected politicians for the rulings of the courts (as far as Germany is concerned, I’d like to make a minor correction of what has been said above: In Germany, only basic principles such as federalism, democracy and rule of law, as well as the right to human dignity, are entrenched, but the other individual rights in the German bill of rights are not). Then again, I suppose that once there is sufficiently wide political consensus in a society to adopt a constitutional amendment that would run contrary to an entrenchment clause, that clause won’t be worth much; politicians and courts will find a way to argue around it. So here, too, the decisive thing is not so much the written constitutional text but rather an unwritten but shared consensus within a society that there are some things which you “just shouldn’t do”.