If an Eladrin can move 6 squares in a move action or he/she can use Fey Step to teleport 5 squares, which is also one move action, what is the difference? If both count as one move action and either one takes your full turn during combat, is there really any benefit to teleporting 5 squares during combat instead of moving 6 squares? Or is Fey Step really only useful for teleporting into a locked area or an elevated position (like teleporting into a tree)?
In almost all cases, teleportation is strictly better than an equivalent amount of movement. This is why teleportation usually shows up at lower numbers than movement for any given power/action type.
Most of the advantages of teleportation are from the fact that teleporting moves you from the start to the end without touching any of the squares in between.
Teleportion is a special form of movement, useful for travelling without movement.
There are two major benefits to teleportation. You can situate yourself in any square in range without provoking opportunity attacks (verry nice for getting at the "back line" of monsters, and you can escape grabs and some forms of immobilized and restrained.
These benefits are not useful all of the time, but they will generally come in handy at least once an encounter, depending on the nature of the space that you're playing in. It is also useful for movement in the z-axis of the game, presuming your DM has prepared for that eventuality.
At higher levels, there are builds that focus on teleport optimization to have virtually unrestricted movement around the battlefield. But in general, the Fey Step racial power is considered quite nice relative to some of the other racials out there.
Teleportation allows you to move when you are restricted via immobilized and a veritable host of other effects.
In situations where your move action is going to be hindered by immobilization, barriers where you can trace line-of-effect your destination, or you want to travel in a direction other than horizontally (provided lack of ceiling), Teleportation is great.
Considering that turns are broken down into your main, minor, and move I personally have trouble opting out of teleporting the same distance in combat when available.
The only flaw I can find with Teleportation is you stay prone when you teleport.