What makes a good support champion

I think the only requirements are that he needs to be effective with a low gold income and offer some sort of protection for other team members.

nothing they all suck,JK.

Depends on the game, each support has their strengths and weaknesses.

Personally I find that champs that have good engage/disengage are great.

Just being helpful. And no, being 3/0 early doesn't count.

To be serious though, you have to have some way to help your teammates, and not just yourself. A hard CC that makes it easy for yoru carries to get kills, a heal or shield to keep them from dying. Even if they can do damage, a real support helps their team with everything but damage.

You just got to ask yourself "what would be most helpful to my team?". Early on, make that question "how can I help my ADC?". It's a very simply question, but if your champ can answer that question, then it's that simple.

Reliable CC and team based utility. Nami for example

Her passive speeds up her allies. Q is long range hard AoE CC that can't be body blocked. W heals/hurts. E helps people stick to their targets. R is a great initiation tool that AoE speeds up your allies and slows your enemies.

Generally being able to work with lower gold income.

Supports are generally divided in 3 categories:

1. Damage
2. CC tanks.
3. Enchanters.

For the first category, the damage dealers are generally the lease effective, because in order to do damage, you need items, and with low gold income, that isn't happening often. There are few champions that have good early base stats to be doing respectable poke damage to make them viable. Those are Brand (% max hp health in burn), Zyra (can take some time to set up her combo and then burst you), maybe Lux with her long range skills early. Generally, I don't like to play, play with, or play against these supports. If I want a damage champion, I'll play a carry position. If I want to support, I tend to Enchanters. Playing against these can feel miserable with certain champions.

The second category are tanky champions that offer a lot of utility, either through CC, protection, or playmaking. For example, Alistar is a tanky support, that has quite a lot of CC in his kit, and is quite tanky from his R. He can also heal to some extent. Braum is a champion who also has some CC and can protect allies with his W and E. Kench can also protect teammates with his W and can also set up plays with his R. These are generally good supports, because they don't really need gold to apply their utility, and they have innate tankiness so they don't rely on buying items to become tanky. Generally, these champions lack in other areas. They are melee champions, don't have mobility, or it's situational mobility and they don't deal a lot of damage. They are quite effective.

The third category is my favorite. Enchanters are the true supports in my opinion, because they "support" the team by enhancing their stats and abilities. My favorite one, Janna, is one of the best. She works on low income (even though more gold on her means she can get to more items which is important), she has a very good Q that denies a lot of things, has the E shield that also grants AD, has her ultimate etc. You generally realize how she enchants her team. Same goes for Soraka, Nami, Sona etc.

Now, what I believe is the best class is a combination of all of these, which is hard to find. A combination of damage, utility and enchanting. Janna was like that towards the end of season 7. She had a lot of damage, while having the CC, utility and enchanting (Ardent Censer, W busted damage etc). Another support that comes to mind is Taric. He has heals and shields and armor buffs to enchant an ally, he has his ultimate as well, while also being quite tanky himself and being able to apply CC with his E. He doesn't have a lot of damage, but he generally outdamages his fellow CC tanks, if he can get in range to make use of his passive.

So that's how the supports work. It's up to you to pick what you feel like you'd have most success on. I found Janna and Taric, because they both have playmaking potential, which is important. The worst thing to have in a team are teammates who are scared to take initiative, or do not know how and when. Playing supports that gives you initiative (somehow I make Janna and Taric work in that regard) is important, because if no one else won't, then I'll have to.

Probably completely unrelated, but supports that apply debuffs to enemies and passive buffs to allies (thinks Bards in MMO). A support shouldn't be a small dog following one character and buffing / shielding / healing him all the time, it should be interactive in both ways : passively offensive (idk how you'd say it, applying debuffs to enemies without slowing / stunning them, like more damage dealt to them from allies, allies healing when hitting them (think old Q)) and also protecting their allies allies from behind by boosting their move speed, applying shields / damage reduction