Re: isolation vs inflection & other features

Pardon me for being paranoid, and I hope not to sound presumptuous, Ebera,
but have you been here before under another name? (Nothing wrong in that I
know - if you haven't please forgive my asking; it's just that your style
looks familiar!) First up, how is 'bikini' genitive? That would be "a
bikini's girl on the beach", but it could be argued that your example,
lacking a verb, is not even nominative either. Additionally, the way you
have written the example (omitting the indefinite article before bikini)
could more properly mean "A girl in Bikini (the island) on the beach".
As to structural differences; surely (by definition) there is a world of
difference between isolating/agglutinating systems. The difference is,
however, morphological and syntactic rather than expressive; and ease of
learning depends on a great deal other than the structure of the language
you wish to learn - your motivation, linguistic background, typology of your
birth-language, and so on. For example, to take your affix versus
preposition model: English speakers, for instance, will tend to find the
French syntax easier to acquire than (say) Chinese, because there are fewer
differences between French and English on that front. However, they will
struggle more with the idea of masculine/feminine nouns and conjugated
verbs.
In your examples using 'at', unfortunately only the first two are acceptable
English, though the second means 'at' and not 'on'.
Prepositions are not unnecessary to those languages that have them; they are
an essential part of the grammatical system. As for your summary, all I can
say is "long live messiness". Human languages are never idealised systems
for the same reason that human beings - or any Conlang with soul invented by
them - are.
Erm... just why should a conlang implement a pure system (whatever that is)?
As for the position of case markers, try telling that to an English speaker;
it's not as though there's a shortage of them. Example - the boy's book vs
the book of the boy. It makes no difference that one case marker is
indicated by a dependent morpheme while the other uses a diferent syntactic
device. The fact remains that the two phrases have the same deep structure.
I could go on, but clearly the reason why English is the most widely-spoken
language in the world is because it is unbelievably messy and hard to learn.
Mike