葉月 said:
IMHO "as close to kana as possible" is a rather worthless feature of a romanisation scheme, it should be fairly close (so as to give some reasonable clue about the spelling), but not so much so as to sacrifice more important characteristics, such as being non-ugly and consistent about pronunciation clues[*] (that's pretty much the only point of using a romanisation scheme; if you want to be really close the original JP text, you should just use the original text and stop being a pussy). Still, I agree with the general principle of being consistent and close to the original when possible.
I disagree - the main reason for romanization is not to give pronunciation clues, but to give a standard representation for a language in the Roman alphabet, because speakers of many languages can instantly read latin characters, unlike, say, kanji/kana. For example, I could easily remember how to spell a Polish word without having any idea how to pronounce it, whereas remembering what different kanji look like is something I find more challenging.
It does help that orthography in European languages is vaguely consistent (a is usually close to /a/, o is usually close to /o/, for example), but look at how for example hanyu pinyin uses the letters x, c, q, etc. and diacritics in rather unintuitive ways (to a speaker of a European language) but to great effect. Of course, it then goes on to do some weird shit with the finals, but hey.
"Consistency within itself" is easily achieved by making sure that spelling follows pronunciation. Other than that, I'm not sure what "consistency within itself" really means, other than that there should be an easy algorithm for spelling a word and for deciphering one.
Consistency with kana not only helps students of Japanese in the transition to using kana and kanji, but it also doesn't conflict with "following the pronunciation". There may be two ways to spell the same pronunciation (づ and ず, for example), but there aren't two ways to pronounce the same grapheme, as there are in some other languages I could name (c in English, for example). This means that if we follow kana, we are not losing any information. Furthermore, there's a nice mapping from kana to latin digraphs: there is a canonical "kana table" which has rows and columns, and it is quite natural, orthographically, to assign digraphs to its entries by mapping the column to the first letter (the "consonant") and the row to the second. One column in the kana table, 「たちつてと」, becomes "ta ti tu te to" in ローマ字. Compare with Hepburn "ta chi tsu te to". Sure, the latter looks like how we'd spell the actual sounds in English, but I find it more "gratuitously insulting" and less "non-ugly" than the former (nihonsiki), to be honest. By the way, this table isn't in any way arbitrary either, e.g. the stem of a verb ending with つ will end with ち (育つ -> 育ち). The fact that pronunciation of the initial consonant in the column I gave differs from row to row is likely a historical morphosis.
Consistency with "general European standards for how certain latin letters should sound" I find arbitrary and a non-issue. As I said previously, any failure in the intuitiveness of the spelling in a romanization system can easily be overcome by the prospective user reading a short wikipedia-style article. ローマ字 shouldn't be a crutch for (and solely the domain of) people who can't be bothered to learn a few simple rules.