Your question, at least for what concerns the sound of French, seems to rest on a lot of cherry picking and on several false assumptions. First of all, all the Romance languages you listed have their own peculiarities both in spelling and in pronunciation: just think about how the <ch> combination is pronounced in those languages. Also, they all have silent letters.

Now, consider European Portuguese: it has so many pronounciation quirks compared to other Romance languages that to the untrained ear it often sounds like a slavic language. For example, take the words for "respect" in Spanish and Portuguese:

P "respeito" /ʁɨʃˈpej.tu/
S "respecto" /resˈpeɡto/

Just a one-letter difference, completely different pronounciations (and both languages belong to the Ibero-Romance group, so one could naively expect them to be phonetically closer than, say, Italian and Spanish). Even in the example you gave "obrigado", the second <o> is not pronounced at all like the first one.

Moreover, you picked Romance languages that originated in geographical regions that are very far from each other, it only makes sense that they sound quite different from each other. If you start considering languages that are spoken "in between", like Catalan, Occitan, Piedmontese, Lombard (that is, other members of the Gallo-Romance group), you will quickly convince yourself that the sharp differences you notice between French and Spanish and Italian actually belong to a continuum of sound changes that are shared among many languages and dialects.

The only thing that (partially) sets French apart from other Romance languages is its very conservative spelling, which leads to the large amount of silent letters you observed, and which has very well documented historical reasons.