I'm a Warne fanboy, but from a objective point of view you could say Warne would have definitely had his moments as a captain.
The problems would arise from if he could get the whole team behind him. He managed that with Hampshire and RR, but in both cases it was probably more due to the fact that he was a Legend and hardly anyone is going to raise issues with that, especially when he was so enthusiastic.
But in the middle of his career, with his off-field issues and some as experienced, if not more experienced, players in his side, there could be problems. Could he manage all those big-name stars? Who knows. But it's doubtful, Warne's a greate people person but the stuff he did off the pitch could easily have lost him respect.
Tactically...I have only seen him in action for RR - faithfully for the last 3 seasons, nearly every match - and yes, attacking is his main mode. He doesn't see the point in restricting runs unless doing so leads to a wicket. That's an admirable trait, and it works well in T20s. He has also a couple of times (the match against Mumbai in season 1 comes to mind) played defensively with bowling plans to get the match into the final few overs, where they have the best chance of a win, rather than going gun-ho.
I don't doubt he'd be a fine FC captain. He seems quite brilliant tactically, reads batsmen very well, and he does all the basic obvious moves right, with the odd piece of brilliance here and there. Does he make the best of resources he has (ala Fleming?). IPL 1 would suggest so, as would his success with Hampshire. Can he lead from the front? He certainly seems the sort. Does he get the best out of the players? Everyone who has played under him says he has.
But would he have been a success leading Australia? Doubtful, considering his offield problems, the big names in the side, and that he wouldn't have the experience he had in the Hampshire/RR stint.
But like I said, he would have had his moments.
Would have been a great captain in his final years too.