Syncope - how big of a concern is that?

Its not a big deal at all if you can see it coming and get to a place where you can recover before you pass out. The only real danger comes from actually passing out in a circumstance where its dangerous - like when driving a car or falling and hitting your head. It might all be a matter of physics. Being tall and thin, its easy for blood to pool in your legs and not so easy to get it pumped back up to your heart. Thin people tend to have a very low capacity to store excess fluids, so your blood volume can drop rapidly if your are sweating or drinking alcohol or caffeine. Staying fully hydrated and keeping your legs moving (instead of standing still) are two things that might help.