I have over 500 hours into the original Insurgency and want Sandstorm to be even better. With that said...
The lean system and vaulting is absolutely ruining the game for me. I lean a lot and to see it so different than Insurgency is very frustrating. The issue is when leaning, if I strafe or move, my aim is forced immediately away from where I am aiming. I guess this is to simulate realism/momentum? But it is not realistic. In the Army I could aim down sights and hold a lean just fine while walking. I see no reason to make my aim flick away for simply strafing or un-strafing. It has gotten me killed many times because I will lean, take a step, and now my aim is completely off of where I was aiming, and when I stop walking my aim again flicks away for a moment. It's punishing us. Make it a smooth, fixed lean without the jerking around when strafing.
To demonstrate/duplicate this, go in-game, toggle lean, and tap W or D to strafe. See how frustrating it is to have your aim flick and jerk around for simply tapping a strafe key? "Squad" is a game that has a realistic and intuitive lean system. I recommend something more like that.
Something else that I wish to see changed but is not vital is the menu system. Whether it be cosmetic or loadout related, having to go through multiple tiers of customization to reach something is very tedious. For example in the original game, adding attachments to guns simply made your loadout list longer and a simple scroll led you to the next options. But in Sandstorm you have to commit to a menu category and constantly keep clicking "Accept" to back out and customize another item.
Thanks for acknowledging the feedback and I offer it to make Insurgency a better title.