We’ve got an apple tree at the bottom of our garden. Because I have the gardening abilities of a small pickax, the base of the tree is surrounded by weeds, many of them big and spikey.
The lower boughs I can reach. The fruit there is invariably half eaten, by birds, by maggots. I’m convinced even the foxes have a go.
The upper branches are where the good stuff grows, completely inaccessible until they fall to the ground, over ripe.
If I could be arsed I’d sort out the sorry mess at the bottom of the tree. Then I could get a ladder out. But there’s too much else going on, and quite frankly I can buy apples at Sainsburys. Not the attitude, I know, but I live in London not Somerset.
Remember this the next time someone is talking to you about the project going after low-hanging fruit. It’s invariably inedible, and if you can’t be arsed then go to the supermarket.
mmitII 
Doesn’t that mean the metaphor works well when applied well? For example when transforming an organisation’s datacentre services – low hanging fruit might be the mail servers and other such stuff that’s a pain to manage and easier/maybe cheaper to buy as commodity from a SaaS provider (i.e. from an external marketplace)?