Having some sort of concealment via stealth to ambush and crowd control is an absolute must. Ranged builds have the potential to deal a lot of damage very quickly, but they're very vunerable when swarmed. Unless you switch to a melee weapon/shield, but that takes away a lot of your DPS.
Darkness, summons, animal empathy, traps, or anything that can slow/immobilize an opponent (epic traps, death attacks, etc) are all needed. The more options you have the better. Called shot: leg is pretty much a requirement to prevent things from closing in on you as well.
Anything that can boost ranged damage should be considered. Standard Assembly templates, divine might, sneak/death attacks, and certain poisons and spells can all be of use, though each has their own issues. Everyone should make use of templates, but they're another consumable resource and can get quite expensive over time. Divine Might requires CHA and divine class levels. Sneak/death attacks require getting close to a mob, and absolutely puts you in "get swarmed" range.
Ironically, the greatest danger to me when I played a ranged build was other ranged enemies. Mages typically don't have great HP or detection skills and went down quickly when ambushed. Melee enemies could be immobilized, kited, or worn down to critical health before they got into melee range. Ranged enemies though? They typically had a detection range farther than what I could physically see on screen, and had an AB high enough to punch through my AC while I ran for cover.
I haven't tried horse archer builds or the new AA's, but I the same basic strengths and weaknesses would still apply i'd imagine. I actually would -not- recommend a horse archer, as they require mounted feats to use effectively I think, and its going to be a waste when trying to explore the 60%+ of the server that is inaccesable to horses.