Its simple, so simple actually. All you need to do is create faces which people can relate to and get behind. That have character flaws and struggles like any other person goes through. Whilst Austin was a bad ass the main reason why I think he got over so well was because he came across like an every day guy who was going through having his boss treat him like shit. And Austin's character had flaws: he didn't take shit from anybody, he didn't respect authority which we are taught to do from a young age (see school), he had an attitude problem which led to him doing things which in reality, he shouldn't be doing but wanted to do. These are all things we can relate to in some form or another.
The reason why faces like Cena, Sheamus and Kofi aren't as great babyfaces as they could or should be is because at times they don't even come across as human. They don't react to bad situations, they don't get really mad or upset, or do things in a reaction or a desperate attempt to rectify things. There is no sense of a struggle to the top and no human characteristics which make people and me in particular want to get behind them like I did with Stone Cold or Rock. A big part of it as a few people mentioned is a lack of attitude behind the characters. Cena, Sheamus and Kofi are huge examples of faces where at least half the time they don't have a sense of attitude. Everything is happy go lucky and they are pretty much a goody too shoes. Can't do anything wrong at all. Its boring.
You can't invest in a character like that.
People are going to hate me for using this example but its so true and its a recent one, so it shows the impact of what I'm talking about. The reason why AJ right now is so over casuals is exactly the reasons I am talking about. She isn't your typical barbie whole girl like Kelly Kelly who was only popular for her looks. She looks like someone you could hang out with in real life and the things she's been through people can relate to and get invested in as far as her character is concerned. Like for example, her relationship with Daniel Bryan and how he treated her and then subsequently dumping her. The fact that she gets easily attached because of unrequired love and the fact that she feels no one loves her. Even the current storyline as horrible as it is shows an example of what I'm talking about because of her being falsely accused of something which cost her her GM role and the way she is reacting to it all. She is also not a character without flaws, she has volatile mood swings and can act like a real psychopath. Her snapping at Kaitlyn several times after she got dumped by Bryan is great example of this. She could have easily been portrayed as a heel after that but her actions generated a lot of sympathy for her. Her character is very real
(regardless if you think the execution of it is bad
) which is why the majority of fans are latching on to her.
If WWE created more faces that were more like actual people with struggles and flaws to their character instead of trying to make ones that are super human and can't do anything wrong, we would have better faces to foil the heels. But alas for now, I'm mostly routing for the heels. At least a lot of them have something
I can relate to and get invested in.