Applejack is a hot-headed, narrow minded and honest pony. Meaning she is quick to jump to conclusions and answers without thinking twice about it. She is very similar to Rainbow Dash but different in the regards that she embodies what her element is, Honesty. Where as she wouldn't back down on a challenge, like Rainbow Dash, she would never try to find shortcuts in order gain leverage over someone and win. This is what people don't get about honesty. It is not about just telling the truth, it's about doing things in a right, fair and modest way. Where as Rainbow Dash in the newest episode would lie and avoid the truth to Pinkie Pie about pretending to eat her pies, Applejack would just straight up tell her that she does not like pies. In the episode where Applejack and Rainbow Dash races, Applejack never once tried to gain an advantage by doing things outside the rule book and only when Rainbow Dash initiated the rule-breaking did Applejack see it fair to do it as well. Applejack likes to play on an equal field so that everyone has a chance, including herself.
Applejack is also very quick to anger and she sees things black and white most times. She gets angry when Apple Bloom doesn't want to ride in a traditional cart and she gets angry when people insult apples. She is very 'simple' in her way of thinking. She takes things precious to her very personally. Only one who is more aggressive about this is Rainbow Dash. So Applejack isn't much about compromising and it's either all or nothing.
Also, I wouldn't call Applejack smart. She might actually be the least intelligent pony among the mane6. But she does understand when ponies are out of their comfort or when someone is hiding something, we can see this with Scootaloo where she has nightmares. Applejack is the only one who consistently asks her if anything is wrong. She does this a lot with Twilight in the earlier seasons. She is almost always the first one to ask if someone is in trouble.