Cast on Ignite build

You're pretty much invincible to anything that doesn't one-shot you.
You can run almost any map mod, (certain combinations can of course be difficult).
Doesn't rely upon leech.
Assuming that you balance the mechanics right, you can just about walk across the map and wipe out everything on the screen via autocast without having to do anything.
You can AFK most content. This is especially useful when my cats walk across the keyboard in a bossfight and pull up the menu.
Trolling parties.
Trolling GGG's servers.
Looking amazing, as long as you're rocking a 5,000$ computer.
Rocking a dozen spell MTXes simultaneously.
Makes Discharge and even old-school CoC look like wussies.

Hardcore. For real. Any build that does several thousand SELF-DPS is not a hardcore build.
You have basically no mitigation, so one-shots are (even more of) a concern.
Since one-shots are the only real threat, and you can't do anything to mitigate them, certain content is strictly no-go. This build is never going to be an Uber Atziri farmer.
Volatile Flameblood & Bearers are massive threats, and hard to see due to the amount of spam on screen.
Life flasks are not a thing. If you're going to die, you're going to die instantly. I keep one to deal with DOT and some other cases like no-regen maps when enemies aren't around to get life off of, but that's it.
Relies heavily on non-life uniques. Most are cheap, but it means that you're leaning on the skill tree for life, so you'll struggle at lower levels.
It's got pretty decent clear speed, but nothing to write home about. And boss fights are a bit slow.
FPS and ping. Do not bother with this build unless you have a decent rig and a good server connection. You can sometimes disconnect from the server or crash the game even with a great connection and box.
The build is 100% socket-starved. I hate not having a portal gem. Also, no movement gem.
A lot of the final high-end gearing relies on self-rolled Shaper items and corrupted uniques, which can be tedious and a currency sink if you get unlucky.
Needing 0 armor makes it a bit harder to gear since about 1/3 to 1/2 of gloves/boots will have it, and you have to avoid armor rolls on the belt.

I'm running a (legacy) Rat Cage, which not only lowers my max fire res to 70, but increases my fire damage taken by 20% and causes me to take 10% of fire damage as physical damage. At 15 ignites per cycle, that's 15*100=1500 incoming fire damage. 10% is taken as physical, so 150 phys (no armor), plus 1350 fire damage * .3 (resistance) * 1.2 (20% increased), for 486 fire damage, or 636 total. Add to that a 64-mana cost Incinerate (so lame, not even 21/20...) @ a cool 7 casts per second and that's 64*4*7*.19 or 340 more phys damage per cycle, or 976 total damage per cycle (DPC) to yourself. In return, you (instantly) recover 2700 life back, netting yourself a cool 1724 total life swing. Per CwDT cycle. Single target. Conservatively.

When you sit on top of a boss, you're getting way, WAY more ignites per cycle. To see why, here's my (current) gem setup:

Items:
Note that you should avoid having ANY armor whatsoever on any item.

Hands: 2x Razor of the Seventh Sun (required); any roll will do. Only 3s so coloring isn't an issue even though they're off-color. Ultra-endgame: Corrupted for life leech. Preferably fire, but probably any will do.
Helm: Scold's Bridle (required); high spell damage and/or mana rolls are nice, but not a huge deal until you're ultra-endgame gearing. For perfect gearing, a good enchant would be nice. Preferably something that could give you more ignites. But since we run so many spells, it's an extremely minor benefit.
Amulet: Eye of Innocence (required); The only roll that matters is the str/dex, but it's rollable with blessings, so not a huge deal. Or corrupted for mana on damage taken.
Rings: 2x Ngamahu's Signs (required); The primary roll is recovery -- it should be perfect (30), with added fire damage being important as well, and str being a nice bonus. Again, the implicit is important but blessable. Or corrupted for mana on damage taken.

Armor: The Rat Cage is highly recommended, though not required. It gives high life, movement speed, increased fire damage taken (both as a mod and via the fire -> phys mod due to lack of armor), and if you get a legacy version, it can lower your max fire res. The non-legacy version is viable, but not as good due to no lowered max fire res and having -fire res, which must be made up elsewhere. Also, all versions of The Rat Cage will strain you on dexterity requirements, which must be covered via gear and/or tree points. You can, however, go for a rare with high life (and res rolls). You do not technically need a 6l. A 5l will do, with Anger swapped to the 6th (unlinked) socket, and Magma Orb moved to Anger's old socket. However, if you can do a 6l, it's better due to the GMP being linked to an additional spell.
Gloves: The only truly free slot. High life, resistances and/or dexterity (if using The Rat Cage). Depending on your other gear, this could be cheap or expensive.

Belt: For lower-cost versions of the build, another free slot. Again, high life and resists. Obviously, Leather Belts are preferred here. If you've got money to sink into it, though, an ilvl 85+ Shaped belt can be rolled to give +20% cooldown reduction. Then just add a high life roll and as much resists as possible (move speed master mod is nice if you don't have to master mod resists/life onto it).
Boots: For most people, this is just another free slot -- high life, movespeed, and resists/dex. For those who have their +20% cooldown belt, this is the next piece -- a Shaped +15% cooldown reduction mod, high life roll, movement speed, and as much resists/dex as you can get.

Jewels: 3x Fevered Minds is highly recommended, as it helps with both damage and mana cost for CwDT and Arcane Surge proccing; perf rolls obviously preferred. Some variants of the build might run 2 or 4 of them, though. The other 6 should be 7% increased maximum life + 5% chance to ignite; they should also have a third (or 4th if you're filthy rich) mod, depending on what you need to make up for after you're geared. You can go elemental resists, spell damage, or even stats.

Flasks: Wise Oak is pretty boss, but it's a pain to balance the max reses to get the fire pen on it (you do deal some of every element, so it's still good even without it); additionally, you could set your fire res to be the lowest to protect against heavy fire damage enemies, but it will slow down your CWDT cycle. Obviously a movement flask is nice (I like 2 Rotguts personally), and you need the standard staunching and thawing somewhere (I've been getting away with no dousing). While your cycle still runs even if you're frozen, you don't want to be locked in place and unable to dodge exploding lockboxes/Flameblood/Bearers. Grounding isn't a bad idea given that it helps avoid one-shots. One life flask is also not a bad idea just in case of emergencies. All of the non-uniques benefit from Surgeons since, while we don't invest in crit chance, we spam a bucketload of spells, so we generate crits pretty consistently.

Note on CwDT gem levels:
Higher level CwDT will give more DPS, but less ignites (defense, spam amount). It's really more of a preference thing, but getting decent clearspeed on high-level maps will require higher CWDT levels. The main thing is to stagger your gem levels -- personally, I put the spells that proc more ignites as lower level, and single-ignite spells as higher level. This helps with sustaining the cycle procs, though some of them (like Volatile Dead) would also be well-used as DPS. Since the gems on cooldown don't count damage taken until they come off of cooldown, having gems different levels means that (in theory) they will come off of cooldown at different times and thus start tracking damage at different times. Also, and this should be obvious, quality CwDT gems (and quality versions of most spell gems, for that matter) are a good idea. A 20q CwDT will give you ~2.5% more damage.

Quality on the spell gems will vary from 0% (increased AoE/projectile speed, which are still good things to have even if they aren't technically DPS) to 5% more damage, but again, they're relatively minor investments, so why not? One note is that Bladefall might be better off without quality, as lower AoE makes it more likely to ignite a single target more times, and Fireball will give no benefit with quality if you already have 100% ignite chance.

Corpse spells: Pre-3.0, I didn't use DD, but once Unearth/Volatile Dead/Cremation came out, it became incredibly efficient to use them due to multiple ignites per cast (plus Volatile Dead orbs are seeking, which helps with clearing spread out enemies and sustaining the cycle between packs). Unearth is crappy at hitting enemies, but even if we assume it never hits them, it + Volatile Dead alone is totally worth it, and Cremation+DD are just icing on the cake. They do, however, require standing right next to enemies, so if you don't like getting in their faces, you should probably remove them (and the GMP in the chest, as it's mainly there for Unearth).

Don't kill yourself! No, seriously, Scold's can kill you. Learn to stop hardcasting when a boss is almost dead, or you might die if they hit you just before they die and your finger is holding down that "deal massive damage to myself!" button without an enemy to get the life back from.

Try to keep your best pack-hitting spells on a lower-level CwDT so that you can literally just run through the pack and self-sustain the cycle. That is a bigger clearspeed boost than anything else you can do with the build.

Again, this is unfortunately a very lategame-centric build as you need your ignite jewels and nodes, plus a ton of life nodes. 4k is enough for white maps, but 5k+ is what you should look for with yellows, and reds are more like 6k+. Depending on your gear, etc, I would peg that as roughly 70-80 for whites, 75-85 for yellows, and >80 for reds.

Ascendencies:

Gear: (note that I am assuming 7000 life and 42 damage taken on ignite for life gain calculations in this section)

Alternate skill gem discussions: