Comments along the lines of 'music is my life' usually from a male - not always true, and not always just men guilty of this, but this sort of line is common from men who do not want to commit and thus will string you along or use you; they're telling you from the start that you will not be the most important thing in their life and so allowing themselves to act like jerks.
No respect for privacy - if they check your emails, IM's, ask about your mail, where you're going either when leaving the house or just going to the bathroom - they may be paranoid or overly protective, but that in itself is not healthy and something they need to chill about...with us aspies I think this is particularly unhealthy as we like our privacy and have too much weird behaviour we can't explain, lol.
Forcing you into anything - it goes without saying but it can start subtle and this is often how abusive relationships start, it's all about control. It starts as a bit of light-hearted talking you into doing something, or them maybe starting an argument or mentioning that you may not love them right after you say no to hugging or having sex with them.
Negative comments - I got comments like 'you're too thin, I like a woman with something to grab onto' from one ex or 'I wouldn't respect you if I did that' from another when suggesting something sexual, if you're not in a good place yourself or don't know that this is actually insulting then you will tolerate it when you shouldn't, so watch out for back-handed comments, criticism when not asked for, sexist comments that try to control your behaviour in a certain 'acceptable' way, or just outright insults.
Hitting you - another 'goes without saying' but abusive relationships tend not to start off with rape and beatings straight away, it's subtle, they do it once then apologise you love them or are dependant on them so stay and even if it doesn't progress the fact remains that they did this at some point, it was abuse, it's not a healthy relationship.
Cheating - I don't believe that because a person cheats they are a bad person, often people will cheat because they are unhappy with the relationship; thus unhealthy relationship, but it can be worked on and a couple can last a long time despite one or both of them cheating earlier on in the relationship. There are however those who cheat because they have zero respect for you.
Passive aggressive behaviour - they'll agree to go with you to an event but point out how much they'll dislike it or make a point to make you late or not turn-up when planned, they'll sit back while you do something wrong then make a point of pointing it out to you.
There are probably more, and better examples, god knows I've been in plenty of bad relationships...some good. Most things about unhealthy relationships you really don't see, the important thing is to have other people around you to tell you when something is wrong - talk to friends about everything in the relationship, even the stuff you're ashamed of (if ashamed, there is a warning sign!) so they can give an impartial view of the relationship and help you get out if needed. It's normal to lose friends when you go into a new relationship, but if a partner tries to stop you seeing friends or takes up all your time, there is another warning sign.
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Bloodheart
Good-looking girls break hearts, and goodhearted girls mend them.