I'm having a lot of fun with Alliances when they go right. The rewards from agreeing not to blow each others faces of are very much appreciated when you're on that last push to legend.
My only problem with the system is that there is no penalty for a ship pretending to want an alliance only to then ambush you as you attempt to join them or immediately after you have joined them.
I solo sloop a lot, so I tend to stick my alliance pennant up in this circumstance to avoid confrontation whilst I grind out a ton of rep. When you are on your own, being fired on and chased from an outpost or island by a larger ship (or a duo sloop) can be difficult, particularly when the other party has voiced their intention to play alongside you. Using systems from other games as inspiration, I think it would be useful to enrich Alliances with the following tweaks:
If you are in an alliance and one of the alliance ships opens fire on you (let's say twice in quick succession to mitigate accidental firing) their flag is automatically taken down from their ship. As such they are soft marked as hostile to you and you can run like the wind, knowing that they definitely don't want to be your friend anymore.
In extreme cases of griefing, a temporary reapers mark could fly from an enemy ship for say 5 minutes, allowing anyone nearby who does not want to fight the opportunity to adjust their course appropriately (or give PVP players a marked target to chase down as per the standard reapers mark system).
Of course, I know we all play this game differently - and for different reasons - so I expect that not everyone will agree! :)
