Stronger the product owner, better the agile adoption. Product Owner should be able to work and manage stakeholders, team collaboratively. He should be strong enough to act as filter for stories that go in sprint to generate maximum ROI.
As scrum team,teams should have ownership and liberty to pick stories and work on them. Since single requirement in broken to multiple user stories, and multiple teams working on them in parallel there could be chance of deadlock.
Since, scrum enforces continuous integration at end of each day, automated suite for regression ensures that nothing has been broken in process of integration.
When team has ownership and liberty to pick stories, to work they directly become accountable for sprint success and failure. Hence with ownership comes accountability...
If agile implementation hits roadblock, it can only be over come if you have real support of upper/top management and they really want to "go agile". You might have plans or ideas but without leadership support they cant be reality
Last but not the least teams should be well integrated to run sprints successfully, with multiple teams working simultaneously, stepping on toes can be very frequent. Transparency and visibility within and outside teams should be the key. Like functional tests driving unit test.
I hope this helps you to take one step forward to be agile...