I'm not prior enlisted but here's my take. There will be pissing contests anywhere you go, but I certainly wouldn't categorize them as common, especially among the Lieutenants. There may be a blue falcon but the LPA generally looks out for themselves rather than trying to one-up each other. Regarding higher, you'll get your due respect as a prior enlisted Marine and you'll likely be a step ahead of the boot Lts in many ways that seem like common sense to you, however in many other ways you're still the new guy too. Accordingly you'll have to eat a sh!t sandwich or two (weekend/holiday duty, collateral duties that take as much time as leading your Marines, etc). Sometimes your Marines will have to also. That said, you'll have plenty of opportunities to deflect BS. And occassionally your Marines will see it and they'll appreciate it just as they did when their squad leader or platoon sergeant did it.
Your responsibilities as an officer will be very similar to those you had as an NCO or SNCO and then some: lead your Marines and ensure that they're taken care of. Without going on and on, my platoon sergant once told me (paraphrasing), "Sir, it's your job to make sure we get things done, it's my job to make sure you don't step on your crank in the process." Yes, that's cliche, but the point is we both knew our respective roles and we got along great. There were things he did, and there were things that I did, but the key thing is keeping each other in the loop (that mommy/daddy thing). You'll figure it out.
Good luck in whatever you choose to do.