This a growing list that I'll add to once in awhile. My personal opinion only.
1. He shelters you from your friends or from meeting new people
2. He is moody. He has emotional mood swings.
3. He cannot be bothered to focus his attention on you during scening.
4. He does not believe in limits in SL and is color blind when it comes to safewords.
5. He loves taking you to laggy fuck sims all the time.
6. He spends an inordinate amount of time in SL. Being well-rounded means having a sane balance between your SL and RL lives. Problems in RL sooner or later manifest themselves in SL.
7. He wants to collar you approximately one hour into a conversation. This either means you are the greatest thing since they invented California rolls or he has really low standards. Take your pick.
8. He has a quick turnover of lovers, sub, slaves, or has a huge harem whose members go in and out all the time.
9. His avatar looks like a noob. If he can't be bothered to improve himself, why do you think he would even bother to try to improve and help you?
10. His definition of slexing is restricted to jumping on poseballs and uttering one liners until he gets his wank on.
11. He loves boasting about his conquests and BDSM skills.
12. He has no payment information on his avatar. This either means he has a main somewhere that he is hiding from you, or he has manna falling from heaven (in which case, you had better jump and marry this guy)
13. He likes throwing you way past your limits and later says it's because he's teaching you new things and helping you grow.
14. He thinks you giving an opinion or constructive suggestion is topping from the bottom or you being a bratty sub.
15. He loves tagging himself as a Master, or naming himself as one.
16. He puts down other Doms for no solid reason...other than to elevate himself.
17. He thinks of you more as an experimental subject or wank toy rather than someone who can learn and grow with him.