Mandarin phonology is, to say the least, complicated.* For this reason, most romanizations don't do justice to the peculiarities of the language; what's more, most native speakers, even scholars, never really master the ability to phoneticize their own speech. Romanizations are traditionally done by Western linguists, who have a lifetime's worth of experience phoneticizing languages, except that they must do it in Western phonetic scripts, which are ill-suited to the complexity of Mandarin. Bopomofo has become much more popular in recent years, helping to make things like text input, or learning new vocabulary, a lot easier.
Mandarin has three basic plosives: /p/, /t/, and /k/. Although in Pinyin these sounds are represented as "b," "d," and "g," respectively, unlike European languages (and unlike Japanese, for that matter), these letters are not voiced and have no voiced correspondents.
Mandarin is rich in affricatives and aspirations. Those three aforementioned plosives can all be aspirated:
/ph/ (represented as "p" in the Pinyin romanization)
/th/ ("t")
/kh/ ("k")
The language also has a whopping ten affricatives, the subtleties of which are difficult to communicate in HTML. Basically, they are:
The Alveolars:
/tsh/ ("c")
/tshz/ ("ci")
/ts/ ("z")
/tz/ ("zi")
The Retroflexes:
/tS/ ("ch")
/tSr/ ("chi")
/dZ/ ("zh")
/dZr/ ("zhi")
The Alveo-Palatals:
/tç/ ("q")**
alveo-palatal /dZ/ ("j")
Mandarin also has its share of fricatives:
/f/
/H/ ("h," though stronger than in English and almost like the "ch" in "Loch")
/s/
/S/ ("sh")
/Z/ ("r" when at the beginning of a syllable; retroflex; "r" is a letter which, as we'll see in a moment, does double-duty)
/ç/ ("x;" alveo-palatal)
Mandarin has two glides, /w/ and /y/; two liquids, /l/ and retroflex /r/ (also represented as "r" when not in an initial position); and three nasals, /m/, /n/, and /N/ ("ng"). All in all, that's 29 basic consonant sounds; compare that to English's 24.
The language has nine vocalic sounds (eight vowels and the retroflex syllabic /r/) which can combine in various ways to form diphthongs. Also, Mandarin has a syllabic /z/. These syllabic sounds are rather difficult to represent in HTML. In fact, they're difficult to represent at all; even the Pinyin transliteration is maddeningly complex, and a full discussion would be well beyond the scope of this writeup (and my knowledge).***
On top of all this, Mandarin has four tones, or voice pitches, at which one must say a particular syllable in order to communicate meaning. In Pinyin, the first tone is a high, even pitch. The second tone begins low and rises (similar to what an American English speaker would say if he were asking a one-syllable question); the third tone begins at a normal pitch, but then drops to a certain point before rising back to normal (similar to what a British English speaker would say if he were asking a one-syllable question). The fourth tone begins high, then falls sharply to a low pitch. Some syllables vary in pitch or alter their pitch, depending on the tone of the preceding syllable. Also, there is a "neutral tone" which is unstressed and therefore has a basic mid-level pitch.
Suffice it to say, an alphabetic representation of the Mandarin language would have many more symbols than the standard Western alphabets are capable of furnishing. No wonder they use ideograms!
* Sadly, no, I don't speak the language. But I'm fascinated with phonology of all languages, and there just happened to be a dearth of information on the Internet in general, and Everything2 in particular, on the subject of Mandarin phonetics.
** The difference between the retroflex and alveo-palatal consonants is probably best described like this. The retroflex /tS/ is like the "ch" in the English word "chop," except that the tongue is curled a little bit more, touching the roof of the mouth almost straight up. The alveo-palatal /tç/ is kinda like the "ch" in "cheep," except that the tonge is pushed a little bit further forward, so that the affrication is somewhere between a "sh" sound and a silent "y" sound. Native speakers of Prince Edward Island English (my native accent) pronounce a similar sound at the end of words like "night" and "heat."
*** For the sake of completeness, the eight vowels to which I referred are: the "a" in "father;" the "a" in "about;" a sound somewhere between the "u" in "fun" and the "u" in "fur" (such as it is); "e as in "get;," the letters "i," "o," and "u" as they are pronounced in Italian; and the "ü" sound as it is pronounced in German (or "u" as it is pronounced in French). Some people include a ninth vowel, the "u" in "fun," but for most speakers under most circumstances this will coincide with the schwa. Some of these sounds are represented by two or more combinations of letters in Pinyin, and each letter may mean a different sound in different contexts; the letter "i" is used to represent at least five different sounds, depending on the surrounding letters. I've therefore ommitted discussing vowels in too much detail.
Thanks to Gritchka for helping me with IPA and vowels, and Chattering Magpie for helping me with tones.