The Leviathan was powerfully influential because it articulated questions that still occupy us. How can we justify the state? How far does its authority extend? What obligations do states and citizens bear? In many courses it is situated at the start of the modern canon because it is where our considerations of the modern state begin. John Rawls called it the single greatest work of political thought in the English language (source).

I would say that it did not end up as a manifesto for a few reasons.

• It aims to be descriptive more than prescriptive. It focuses on how reason would lead people to agree to a social contract rather than, say, how a political entrepreneur should instigate change.
• It takes a systemic view of political society. It doesn't serve as a guidebook tuned to the perspective of just one actor or set of actors like Machiavelli's The Prince.
• It serves to justify existing government rather than convince readers of the need for something new. Hobbes looked upon two civil wars in England between 1641 and 1651 and viewed their turmoil as something to be avoided even at great cost. An authoritarian ruler is far preferable to a state of war: "sovereign power is not so hurtful as the want of it" (source). In this sense, The Leviathan is more anti-revolutionary than revolutionary.
• It explicitly disagrees with the practices of a lot of authoritarian rulers. Obligation to the sovereign only lasts so long as the sovereign upholds the social contract. If the sovereign starts violently oppressing the subjects, then the social contract is void. Individuals are also free to disobey laws that would have them put to death, including compulsion to fight in war I believe. So, even though The Leviathan might justify authoritarianism broadly in theory it would probably often undermine it in practice.

edit: added a source

Ah ok. Makes sense.

Your last point though - what about "he that complaineth of injury from his sovereign, complaineth of that whereof he is himself the author and therefore ought not to accuse any man but himself" http://books.google.co.uk/books?id=L3FgBpvIWRkC&pg=PA90&lpg=PA90&dq=Leviathan+%22he+that+complaineth+of+injury%22&source=bl&ots=uxTfEoYFoJ&sig=I4hKc4eIdd0RQGqnExapPgRPRN4&hl=en&sa=X&ei=jKRCU-kapZ_sBpSJgbgK&ved=0CDIQ6AEwAQ#v=onepage&q=Leviathan%20%22he%20that%20complaineth%20of%20injury%22&f=false (sorry I don't know how to do a source properly)

Tyrants have justified their rule on far shakier justifications...