Sounds like a simple idea, but the possibilities are boggling. I'll start with the obvious stuff. From now on, I'll refer to teleportation-immune items as NT items (No Teleport).
More class-based tactical thinking. NT items will be much more popular among low-magic classes, as they don't do much teleporting anyway. Archmages who use probability travel constantly will likely find NT items much too annoying to bother with.
Teleport becomes a less attractive escape move. Right now, teleport is a get out of jail free card. With NT items, teleport is in no way nerfed, yet players will think twice about teleporting. Amusingly, this will produce many deaths; in a game with permanent death, if you're ever in a situation where you're thinking about teleporting to save your life, don't think again. Just teleport.
Fleeing the level is heartbreaking. If you've had to teleport away from a dragon infestation and left an NT item on the ground, you'll want to think long and hard about leaving the level and letting it reset. I predict a lot of deaths from ill-advised attempts to reclaim lost NT items.
None of the above makes the game harder. It just sneaks in some tough decisions for the player to make. Of course, ignoring NT items is always an option. Now, how about some less obvious applications of this mechanic:
Dangerous tempations. Make a vault which includes an easy-to-reach room containing some awesome NT item. The easy way into the room slams shut once you enter, and the only other way out is a nightmarish gauntlet. Hmm. Teleport out of the room and leave the NT item behind, or brave the gauntlet in order to keep the item? Many variations of this idea are possible.
Test of perseverance. Make an optional zone where the only way from one level to the next is by using a portal on the floor. If you want to make it to the end, you'll have to leave the NT items behind.
How about some puzzles:
Obelisk guardian. Make an enemy that guards an immobile magical obelisk. He wears a crown that is magically linked to the obelisk. When in close proximity to the obelisk (say, on the same level), the crown grants the wearer 100% physical and elemental resistance. This enemy will be nearly unkillable. The trick is that the crown is an NT item, so you teleport the enemy and the crown drops to the floor.
Indestructible chest with no key in sight. The key is an NT item, and the key is in the chest. Teleport the chest someplace nearby and the key will remain behind.
High-pressure plate. A vault door requires that the player trigger a pressure plate. Unfortunately, the plate only gives under a thousand pounds or more of pressure. On the level is a mighty giant with a ball and chain attached to one leg. The ball is an NT item and heavy enough to trigger the plate, but if you kill the giant and take it, of course you'll be too encumbered to move. So you lure the giant to the pressure plate and either kill him or teleport him while he's standing on it. Either way, the ball stays on the plate and the vault door stays open.
Double quest reward. An npc asks you to retreive some powerful item from some deadly dungeon. The item is good enough so that you're tempted to just keep it instead of turning it in for the reward. Fortunately, it's an NT item, so you turn it in to the npc, get the reward, and then teleport the npc across town. The NT item, of course, is left behind for you to reclaim.
In addition to NT items, add a new scroll to T4: a scroll of phasing glyph. When read, it allows you to target an adjacent tile on which to inscribe a phasing glyph. The glyph stays where you inscribe it until you or a monster steps on it, at which point it phase doors you or the monster. The glyph then disappears. If you inscribe the glyph directly underneath you, you immediately phase door. If you inscribe it under a monster, the monster immediately phase doors. With this, classes who have no access to phase door can still deal with the puzzle encounters.
NT items wouldn't be much fun if they got left behind by recall or using the Orb of Many Ways, so maybe the description should read something like "Resistant to lesser forms of teleportation." The NT flag could be applied to artifacts and egos as a balancing measure, or just for laughs. (My first mental image of this idea involved an NT metal cap. The wearer disappears with a *poof* and the spinning cap, Looney-Toons-style, hangs suspended in the air for a second before falling to the floor).
I thought of the above while driving home from work. I'm sure some of you can think of much cooler applications. Let's hear them! Better yet, let's see some vault designs with NT items and portals on the floor and whatever evil twists you can think of
