What was a key issue that caused the colonists to declare their independence from Great Britain?

Answer 1

Probably the key issue was that the colonists felt that Parliament, having been elected solely by people living in Great Britain, should have authority solely over Great Britain. This gave rise to the statement "no taxation without representation" ... the taxes the colonists were complaining about were not especially onerous (one example: the tea that was dumped into Boston Harbor in protest of the taxes on it was actually cheaper even with the taxes than "black market" tea that had been smuggled in) and were designed primarily to recoup some (not all, or even most) of the expenses the British Empire had sustained in protecting the colonies in the then-recent French and Indian War, but the notion that a government body the colonists didn't have the right to elect representatives to could tax them really rankled.

Allowing the colonies representation in Parliament wouldn't have made much if any actual difference (representatives from the colonies would have been very much a minority and likely would have been soundly outvoted by the rest of the MPs) and would have silenced the loudest drum the would-be revolutionaries were beating, but hindsight, as they say, is 20/20.

(Oh, and that tea thing? It's been suggested that at least some of the instigators were profiting nicely from tea smuggling and really didn't welcome the notion of cheap legal tea, so there was more financial than political motive involved ... but without the cry of "taxation without representation" the smugglers wouldn't have found it as easy to convince the rest of the colonists to rebel.)