If we're following the rules, I think we need to ... follow the rules. Fey Step seems risk-free when teleporting to something you can see. You cannot teleport to something you can't see (either due to blindness, or because there's a barrier between you and it). Otherwise the way PCs use the ability is completely different from other the ability works in the fiction.
I'd like the fiction to be close to how players play. In most games, you can count out precisely (or just guess really well) how far you can teleport, and take no time guessing. (Unless that's why it takes a move action and not a minor action. Maybe most of that time is just taken up doing calculations.)
I'm not too sure about this. Fey Step does nothing for Stealth by itself. If you Fey Step more than 2 squares, you're still taking a penalty to Stealth. So I think if you're about to teleport next to a creature they get some warning. (That explains why you don't get combat advantage out of using Fey Step. The teleport isn't really surprising. Teleporting out of a bush to gank someone is just as stealthy as rushing out of the bush to gank them.)
I think all of these are valid, and things that could come up in a game within the rules. Not 100% sure about the partially mirrored window, but all the other ones.
Pickpockets get an advantage, but I don't think they get much of one. Fey Step isn't stealthy, so they're not likely to teleport next to their victim and then run away. Rather, they might steal and then (non-stealthily) vanish. Of course, they can only teleport a distance they could have run faster than, and it's not stealthy. The eladrin thief would need to plan carefully. You can be easily robbed around a low wall (since an eladrin could grab your belongings and then teleport over the wall, while the victim needs to climb said wall, which is going to be pretty slow). That's not terribly different from wanting to avoid bushes along roadsides due to fear of banditry, you just have one more thing to fear.
Suicide bombers would have a field day, but long-lived eladrin aren't very likely to do that. Also, no need. Slip beside the victim in a thick crowd. Drop the bomb. Bamf! You aren't moving fast, but you can easily teleport past the crowd (or part of the crowd). Instead of a really explosive bomb, though, I think an eladrin would use a powerful single-target attack, such as attaching a nasty venomous monster to the victim instead. (Also, does D&D have bombs?)
An eladrin assassin would be pretty terrifying. They can only do it once, but they could stab you with a poisoned weapon and then teleport away. Again, the ability is not insurmountable, since it's not stealthy and it's not fast. It is pretty damn good when there's difficult terrain, hedges, low walls, crowds, or anything else that might stop the bodyguards from taking revenge.
Eladrin archers... ow. Every archer might pick another safe position to teleport too, so when enemies get in their faces they can teleport away. Admittedly human archers would try to do the same thing (depending on training, circumstances, etc) but eladrin can reliably pull this off once each (between short rests). Eladrin archers are elusive. A minor advantage on a flat featureless plain (your opponent can charge as fast or faster than you can teleport), a bigger advantage in difficult terrain, and an even bigger advantage if you can teleport to a room with a small window across the street.
It's a half barrier. You can teleport one way and get trapped. That happened in a game of mine, twice. (I was a player.) We had a very unwise eladrin wizard. One time he teleported across a stream to use Burning Hands on some orcs hiding in bushes, and then (predictably) screamed for help as orcs came from behind other bushes and tried to kill him. Worse, he was on that side of the stream by himself and out of my PC cleric's healing range. *Sigh* In another battle, he teleported into a prison call and burned up some bad guys there, but couldn't get out until he took a short rest.
Presuming an army of eladrin, yes a lot of natural barriers can be bypassed, but you can stop them by using two layers of barriers, or laying some sort of trap, so their inability to get back across the barrier really hurts them. I guess I'm saying that Fey Step is an advantage, and a potent one, but pretty soon everyone else learns about the benefits and weaknesses.