Just registered in order to post here. Terraria is my most played game on Steam at the moment, so I guess I'm a fan. Glad to see that the suggestion box is open.
I've read through the first 10 pages of this thread and got a pretty decent idea what the repeat suggestions are. I thought I'd echo a few of the ones I saw that was more or less in line with what I was thinking, and try to add some new ideas in the mix (or echo the ideas that were listed on pages 11-29):
-Drawbridges. I'd actually like to see this customizable with most materials--pretty much anything you can make a regular bridge out of--and have it controlled by a wired pulley. Length of bridge may need a cap, but maybe not. Being able to build one that can reach a sky island might be a little ridiculous. But maybe something as large as ten or twenty blocks would be nice. Pulleys can require a non-electric wiring system, similar to yo-yo string, craftable by spider webs and iron. The speed of the pulley would depend on the material: wood would be slowest, iron moderate, then some special metal or ore for faster pulleys. (Not sure if things like meteorite should factor in, but gold or platinum probably should.) The purpose for the drawbridge would be to create a door that would be near impossible to breach without climbing. As a side effect, a wall-climbing enemy should probably approach during a blood moon.
-Automatic Doors. Basically what it sounds like. A door automatically opens when you're close, and closes when you're not. Can be crafted like a regular door, but also requires one wire. May also require a low-grade ore like copper or tin to house the wire.
-Hydraulic Doors. You could also call this "space doors" or "supermarket doors." Works like an automatic door. Just requires iron or steel to construct, maybe a single wire. "Supermarket doors" would also need glass.
-Remote Trap Doors. An automatic door for vertical movement. Can also be used in conjunction with pit-based traps.
-Craftable Floor and Wall Spikes. Make it out of iron. If this is already in the game, I haven't gotten that far. Basically, for a pit trap to work, it should have something that can injure the character falling through it, and what better thing to cause environmental damage than spikes? Lava? Sure, but you can't craft lava.
-Craftable Lava. Superheat your thousands of dirt blocks into lava using the Advanced Lava-Maker 2000 you just bought from the mechanic (or the goblin, or some new NPC that you have to rescue somewhere). Make sure you got a bucket to collect it. Then go pour it in your pit trap.
-Elevators. We really need something to supplement the mine cart system for vertical movement. Going down is as easy as jumping in a hole, but going up is as easy as timing your grappling hook just right, which isn't always easy. Like the drawbridge, it would require a pulley and some non-electric wiring. The box can be made exactly like the mine cart is made.
-Mine Cart Depth Toggler. There's probably a much better name for this, but basically elevators and mine carts should pass safely between each other, or rails should be able to "slide" into a wall when the toggler is activated to allow the elevator to pass through without interruption. Mechanic can sell this.
-Mine Cart Depots. Catch a runaway mine cart and set it on a new course where the rails intersect. Tracks change via switch. Depots are basically special mechanical blocks designed for halting momentum. They could be used for anything that should stop an object in its tracks, like a reverse conveyor belt (another great idea suggested by lots of people). They could be crafted from iron and gel, or iron and honey.
-Remote Missiles. Taking a page out of Perfect Dark (the one for the N64, if my memory serves me right), have a weapon that fires a rocket that the player can control at the price of the hero standing still until the rocket hits its target. High damage to whatever it hits (and it can destroy a few blocks if the enemy is too close to a structure or wall, or if the rocket comes in from above or below), but the player must create a bunker if he doesn't want to get knocked into something.
-Remote Bombs and Dynamite (or C4). Set the bomb or dynamite wherever. It won't blow until you right-click the mouse. Note: This is unlike the dynamite traps that are already in the game that cause me so much spelunking grief. These are wireless, but require you to stand close enough to see it, and are used primarily for demolition, not trapping.
-Proximity Bombs and Dynamite. They go off when they detect movement.
-Controlled Bombs and Dynamite. This will probably need a better name, but this is for precision positioning of bombs and dynamite from a safe distance. Can be used with sticky bombs, too. Basically, it works like a fishing pole, but instead of casting bait and a lure, you're casting explosives. The idea is that the bomb won't be armed until you release the line with a right-click. So you can cast and recast until you get it positioned where you want. Crafted like a fishing pole.
-Craftable NPC. I'm thinking specifically robots or rock golems, but I also like what one person said on Page 9 or 10 about using living dummies. The purpose of the craftable NPC (and there should be only one) is to help defend the town during attacks. It should either be the strongest NPC, or it should be a normal NPC that can use two weapons and/or climb ropes to get to higher ground. One weapon can be a projectile (flinging rocks, maybe?), and the other should be a higher-damaging melee weapon. Craftable out of stone or iron, but must also have wire (50 pieces or more), a higher level ore, some glass for its eyes, some kind of electrical conduit for power, or a few stars if an electrical conduit is too much, some gel for its essence, and maybe a summoner's eye for its aggressiveness. Might also require a life potion to ensure that it self-repairs.
-Slime Embiggener. Basically a gun that fires electricity. If it hits a slime, it either scales it up to a higher-ranked slime, or it transforms it into one of several special kinds of slime, or splits it into multiple slimes of the same type. Used for farming gels, banners, or rarely summoning King Slime. It can also be used to summon electricity in otherwise dead machines, or remote trigger whatever switches it hits. It could probably be called an "electric gun," but it would basically be a slime embiggener.
-High-powered Rails. Unless we're given the ability to craft faster mine carts, it would be nice if rails could be given "charge zones" where every rail between A Box and B Box is supercharged with an electric current that can increase the mine cart's speed by nearly 50%, depending on its buff. If elevators are included, then this could also apply to elevator zones.
-Geyser. Can spit a large fountain of water if triggered by something.
-Rotating Blocks. This is mainly for switching the direction of a slope via switch. If that makes no sense (and the more I think about it, the more I realize it probably doesn't), then the basic premise could be adopted to create remote stairs (that change levels depending on its two target landings) and escalators.
-Automated Lights. Basically give lights full control through switches, timers, event, proximity, etc.
-Environmental Radios. Basically craftable furniture using iron, gel (or batteries), wires, and meteorite. Crafting two (or some other arbitrary number) can summon a new NPC, the DJ (or the Newsman).
-Handheld Radios. Basically uses the same items as the environmental radio, but can be used to communicate with an NPC from anywhere on the map. For it to work, you have to make two radios, one for you and one for the NPC. Then you assign the NPC radio to one person in particular. Once assigned, that radio is bound to that NPC, so if you want to talk to more than one NPC remotely, then you have to make more radios. They can express deliver the purchased item via flying piggy bank or though something unique to 1.3.1.
-Working Plumbing. I support all of the suggestions relating to pipes, drains, and so forth. But I'd also like to see sinks and bathtubs operational (well, they can be turned on and off, not actually usable) and toilets flushable. These are cosmetic and stupid, but they make the environment seem more alive.
-Tradesman Uniforms. They may already be in the game, but uniforms matching some kind of mechanical or plumbing trade would be cool.
-Blackout Event. A random event like a blood moon where you spend a night in the dark. All wires are dead and all lights are turned off. Any door set to automatic won't work. May attract a new kind of creature, or make it easier for regular enemies like zombies to spawn inside town borders.
-Bug Zapper. Catches fireflies and butterflies if one flies into it. Can be bought from the merchant. Connecting it to a wire toggles its functionality from practical to aesthetic.
-Spring Traps. Used to catch enemies the same way butterfly nets catch critters. Want a zombie on display in a glass case? Catch it with a spring trap. Just make sure you connect it to the pressure plate or switch with wires. Spring traps can be crafted out of most materials.
-Advanced Extractions. The Extractionator should be able to occasionally find decayed wiring in silt or slush. Decayed wiring needs to be mended into regular wiring. It takes two decayed wires to create one normal one. Decayed wiring can be fixed at a workbench or table.
That's probably good enough.
I had ideas for two new biome-based NPCs, but that's off-topic, so hopefully you guys will be asking for new NPC ideas in a future update.
Thanks for Terraria. It's one of my favorites. I wish it had been around in the Super Nintendo days.