All of us confess secretly that we were very keen about leaving incoming links on other websites. A lot of admins and webmaster do whatever is possible to achieve this.
Is in coming links good ?
No doubt, it is. If it is on a reputed website, such links are considered as an SEO asset
Then what is bad ?
You must be careful. I have seen websites sold to irresponsible players who change the whole theme to something adult or NSFW. That means, your links are now from an adult website. Google does not take this lightly.
Anything else ?
Yes, Google is aware of link spamming and they have made it clear through many official interactions that if you are leaving irrelevant links on other websites, Google bots are able to detect it.