I too think that whether you're a middle child, the eldest or the youngest makes a massive difference to how you are treated and thus your self-esteem. As a middle child myself, i can totally relate to your 5 reasons, and my parents are good parents! I'm the 3rd of 4 girls which just adds to the competition. My oldest sister has always been the brilliant one in our family so alot is expected from her and given to her, as in time and effort. My little sister at the same time seems to receive attention simply because she is the last to do things, so its always a momentous occasion (all teary etc). Even my parents admit that me and my other middle sister were sometimes overlooked because of this, which has made me very jelous sometimes and alittle worried too.
But yes, like you at the sametime i'm glad i'm a middle child, i feel that being a middle child forces you to be considerate of the people around you, plus i feel i'm able to understand alot more about life because of the experiences my sisters have been through, but also get the chance to be the wise one and advise my little sis too. So there are positives and negatives.
Although attention and time is shared and you can't go on as many outing as most other families, i think this is made up for by the amount of support you have behind you and in front of you!