Two major things I see missing often are Honesty and people NOT being their "authentic self" right from the start. If someone says "yes" to everything you suggest you easily become lulled into believing you've met your "soulmate".
Years down the road you find out they hated the activity and only said "yes" because they thought you were "hot" or they did not want to risk losing a chance to be with you.
Too many times people tend to bend over backwards to "impress" their (would be mates) early on. After there is an emotional investment they revert back to their "normal selves". Thus you hear the old cliche "He/She is not the same person I fell in love with" The truth is they never were that person to begin with!
Awhile back I wrote a hub asking "Do we save our best for the beginning?" http://hubpages.com/relationships/relationshipsdow...
It's important to invest a great deal of time getting to know someone before you become "emotionally involved". If you don't then you will find yourself putting up with stuff you never thought you would in hopes for a change that's never going to happen. You also need to do some "introspective thinking" to determine what you want and need from a mate BEFORE you select one. Flying by the seat of your pants will most likely lead to disaster. Chemistry can lead to impulsive decisions. Always use your mind when making decisions for your heart.
Always be yourself and let the chips fall where they may!
Ultimately we're all looking for someone who will love us for who we are.