After testing several methods of igniting, I've discovered a few distinct major issues.

1) The maximum base chance to ignite is 25%.

The text for ignite reads "By default a hit has 1% chance to ignite for 4% of the target's ailment threshold". Initially you might think you can hit a target for more than 100% of it's threshold, but you can't - even if you hit a target for 10,000% of it's ailment threshold and it somehow lives, your base ignite chance is only 25%.

1a) Hitting a target lots of times does not improve your ignite chance.

You have to hit it once very hard to see that 25% chance.

2) Increased chance to ignite is multiplicative with the base chance, not additive.

As discussed above, the maximum base ignite chance is 25%. If you have +100% increased chance to ignite, your maximum chance to ignite is 50% if you hit a target for 100% of it's ailment threshold. If you're not hitting a target very hard or it has a lot of hitpoints (highly resistant rares, bosses, etc), your ignite chance will still be very low.

3) The base damage of most spells is quite low compared to attacks.

If any of you have played melee and hit something with big supercharged slam or perfect strike, you've probably seen quite good ignites! Unfortunately for spells, only Comet, Lightning Conduit, and Flameblast are actually capable of hitting hard enough to even see considerable ignite damage.

A "okay-to-bad" two-handed melee weapon in early maps has about 200-250 physical damage, which gets scaled by the attack gem you're using. For example, Sunder has an approximately 350% damage multiplier at level 14.Fireball does 70-120 at gem level 14 and doesn't have any additional multipliers.

Fireball does hit multiple times, but ignites do not stack or improve the damage of previous ignites.

4) The affix and passive requirements are too high compared to other types of damage.

Your initial thoughts are probably "just click crit and you'll have strong ignites!", but it doesn't work that way. You still have to hit a monster very hard and have large improvements to your chance to Ignite. So you click all the critical strike passives, then the fire passives, then the ignite passives... How do you get mana, defenses, cast speed, or anything else?

The keen-eyed among you will suddenly realize why so many fire skills automatically ignite enemies - because hit-based ignite is a non-starter and the balance team hasn't realized it yet. At some point it was determined that fire spells "should just ignite enemies" and so the bandage of "everything automatically ignites" was placed on more than half of fire skills.

tl;dr - Non-Archmage spells are currently quite bad.

^{Replying that minions are good at Ignite means you can't read and should stop speaking.}