Tunnel vision is the most significant issue.
I gave the example earlier of where a pilot looks when they depart from a GA field. Some pilots have not been told how important it is to maintain an all round look out taking off; they are excused. However, those that have often still fail to do so. Why? Well if they haven't flown for a while they become focused - their vision tunnels on the same aspects of the departure. We all become obsessed with the air speed, the AI and whats happening in front. Now stop for a moment, force yourself every time you take off to look above left and right, and then look again at which ever direction that s** who has just joined crosswind is coming from. Form a good habit. don't just think in terms of the clean up checklist but make a mental note before you start the take off roll that you will look in every direction for traffic. I would go further and suggest it is worth rehearsing the other aspects of the departure. Think about the spot on the runway you will reject the take off if the need arises. Have you ever rejected a take off - it is worth telling AT that you are going to practice a rejected takeoff. Think about what you will do if you have an engine failure after take off. We have discussed that subject many times but how many pilots know at what height they will return to the airfield, how many pilots have thought about which way they will turn? Even before you line up how often do we line up because we are told to do so without a care for the next aircraft to land - it happens, I have been told to line up twice in my flying hours when infact the controller had "overlooked" the aircraft already on late final.
So these are examples of some of things we all "forget" to think about when taking off but which we should "force" ourselves to do every time and which if we care to self critique after the take off will give a good indication of whether we have allowed our vision to tunnel or we are up to our game.