I really don't know why Adam Levine never went solo. The rest of the band are just like his (talented) session musicians and all the spotlight is just in him anyway.
They put out lots more songs along this lines of this love. They put out next to nothing like Harder to Breathe. But HtB was okay, It wasn't amazing or anything. I never got the extreme praise of their very first album really.
I haven't listened to a Maroon 5 album since It Won't Be Soon Before Long. I liked the song Nothing Last Forever on it. Have I really missed anything good since then?
Harder to Breathe is definitely their best song. Their second album had more than a few that I liked on it though, like Makes Me Wonder and Wake Up Call. I liked Misery when it came out too. I can listen to almost all of the album V without having many complaints as well.