Honestly, a bad player is someone who is unable to play their current role/job to an adequate standard, and has no intentions of ever doing so.

Really, though, there's 3 different levels of "Bad" players.

There's people who are "Bad" because they're new and they're learning. Be it the job or the duty. But as long as they're making an effort to try and learn something it's okay and you should help facilitate them.

There's people who are "Bad" because they just don't care. They're the ones that you will often complain about. With the stereotypical "You don't pay my sub" if you try and help them at all... The ones that just don't use skills. Playing Ice Mage or never using Ninjutsu/Iaijutsu. You know, people that are for all intents and purposes just sightseeing while the rest of the party clears the duty...

Then there's the people who are "Bad" because they're toxic. They might be able to play just fine, but they will either berate others or troll because they feel superior or feel the need to do stupid stuff (Like, that really annoying thing where the tank has pulled enough that they and the healer can handle and then some DPS/Healer decides "Nuuuuu gotta pull 15 full dungeons wall to wall!!!" which leads to wipes because it's almost like someone knew their limits and the DPS weren't good enough to bypass them with quick AoE melting rotations...)