Contrast to RP:

RP	GenAm
Phonological level
same number of consonants besides a /hw/ pronounciation at some RP speakers	__
20 vowels (including diphthongs)	16 vowels (including diphthongs)
diphthongs in center positions	no diphthongs in central positions
non rhotic	rhotic
voiceless /t/ becomes 0 in Estuary	/t/ becomes voiced /d/
/t/ pronounced before unstressed vowel	/t/ is not pronounced before unstressed vowels (e.g. "wanna” instead of "want to”)
Phonetic level
different allophones for example /r/ is pronounced near the alveolar ridge	/r/ is mostly pronounced retroflex
"higher”	"Darker” quality because of opening the velum and narrowing of the pharynx which effects a larger resonance chamber
Lexical
differences in certain expressions: example counterclockwise	anticlockwise
__	not as strict at forming new words of existing words (for example using a noun as a verb as in the iron and to iron in both, one could form a verb to xerox for photocopying with equipment of RankXerox)
a number of varieties in words especially new words	__
Spelling differences:
neighbour	neighbor