Regarding that topic you commented about:
1. They're cute when they're struggling to deal with their feelings. The embarrassment, the discomfort, the line they want to cross but can't seem to let themselves past it: seeing it in the face of a tsun is priceless.
2. They tend to have the hairstyle I like best (twintails!) and the colors that I find match them well (red, gold/yellow, blue, sometimes brown). Thank god most don't sport green, which I tend to associate with the introverted braided hair glasses-girl type.
3. They (to me, at least) have some of the more entertaining dramas around due to
#1 . I played a VN with a tsun who hated boys, but then grew jealous of her best friend loving the MC, had several chapters of drama because of that, and eventually becoming the true heroine herself. Then there's
Mashiro Iro Symphony (the anime) where the tsun had feelings for the MC but couldn't compete with the heroine. Or
Daitoshokan no Hitsujikai's Kodachi Nagi (whom I have as a tablet wallpaper). Their 'dere-dere' times tend to be loveable when they finally get around to it.
4. They tend to be more extroverted characters, and thus life is never boring around them. They're always over-the-top whether it be them blowing a fuse over suspected adultery, plotting mischief, or saving the world. I don't recall any proper tsun who just sits at home stewing all season and doesn't make up for it in a big way. Kuuderes can fit this role as well, but I obviously love tsuns best for this.
5. They're almost always flat-chested, and will always be defensive about it. Okay, this one's a bit of a joke just to round up things, but I really don't care much for ultra-extra-endowed women in my VNs; their body balance is usually off. That's a different topic entirely though.