Pesky Pronunciation Rules... Yes, that's how much I dislike them.

ㅅ and ㅆ

• pronounced as 'sh' and tensed 'sh' respectively when followed by the vowel romanised as i or a y-combined vowel.

ㅇ

• silent consonant when it is the initial consonant in a syllable, this happens every time the syllable starts with a vowel sound. Vowels at the beginning of the syllable must always be preceded by this consonant.
• pronounced as ng when it is the final consonant in a syllable.

ㄹ

• pronounced as r if it occurs in the beginning of the syllable
• pronounced as l if it occurs at the end of the syllable
• however, when the final syllable is ㄹ and the next syllable starts with ㄹ, then the two ㄹ should be pronounced as l
• additionally, if the following syllable starts with ㅇ and consequent vowel, then the final ㄹ is pronounced as r, except if followed by a vowel combined with the semi-vowel y
• pronounced as n in the example: 음료수. (So far not sure of the general rule)
• if the at the end of a syllable to which is added a verb ending or particle starting with 으, then the consonant ㄹ is dropped. Example: (들 + 세요 = 드세요)

ㄱ, ㄷ,ㅂ and ㅈ

• pronounced and romanised as g, d, b and j respectively when occurring after a vowel.
• however, pronounced and romanised as k, t, p and ch respectively when occurring at the beginning of the sentence
• pronounced as ㅋ, ㅌ, ㅍ and ㅊ when followed by ㅎ (Example 벡화점 pronounced as bae-k'wa-jeom) 
• (wiki) initial consonants are usually pronounced voiceless. For example, the ㅈ in the word 자 is  typically voiceless, especially as the first word of a sentence. That makes it sound more like che than j to an English speaker.

ㄱ, ㄲ, ㅋ and ㄷ, ㅌ and ㅂ, ㅍ

• ㄱ, ㄲ and ㅋ pronounced as unexploded k when occurring at the end of the syllable
• ㄷ and ㅌ pronounced as unexploded t when occurring at the end of the syllable (ㄸ does not occur as syllable-final consonant)
• ㅂ and ㅍ pronounced as unexploded p when occurring at the end of the syllable (ㅃ does not occur as syllable-final consonant)
• However, if the following syllable starts with ㅇ and consequent vowel, then the syllable-final consonant is pronounced as b, d and b respectively. (Example: 집 and 집에)

ㅂ, ㄷ, ㄱ

• pronounced as m, n and ng respectively. Part of a general assimilation rule within the Korean sound system whereby a non-nasal stop becomes nasalised. Examples: 감사합니다 and 미안합니다

ㅅ, ㅆ, ㅈ and ㅊ

• pronounced and romanised as unexploded t when occurring at the end of the syllable
• ㅅ, not ㄷ or ㅌ, is the most preferred letter for a syllable-final t sound when writing a foreign-loan word

ㅚ, ㅞ and ㅙ

• romanised as oe, we and wae respectively, has in the past differences in pronunciation that have since been lost or being lost in modern (standard spoken) Korean.

ㅎ

• becomes silent or weakly aspirated when occurring between two voiced sounds. Example: 괜찮아요 gwaen-cha-na-yo instead of gwean-chan-ha-yo.

More peskiness

to be added in regards to the rules pertaining to the cases of: -ㄴㅈ, -ㄹㄱ, -ㅎㄴ, - ㄹㅂ

* the changes in pronunciation depending on the following consonant is particularly important in the case of the addition of particles such as 에 since it changes the normal pronunciation of a known word.