It's a pet hate of mine and I refuse to let pax do it. If they are at row1 or 10/11 (A319) 12/13 (A320) I make them place all luggage in the overhead locker. Unfortunately not all colleagues are like me and will allow it usually because they don't want to get into the inevitable debate about why it has to be put in the locker and why it can't be placed otherwise. Some crew just like to avoid conflict but me.......if they want the 'better' rows then passenger must comply with the additional procedures and using other peoples leg room area is unacceptable.
As for it being "your" space I once saw (on Easy) several passengers have their own items handed back to them from an overhead locker and being told to put them in front of their own feet in order to create space for a single passenger's oversized item which would only go in the overhead.........guess the cabin crew had been dropped in it by the gate staff and loading the large item in the hold would have meant a delay.
I totally agree with the small items under seat procedure and on a busy flight I will actively enforce it. As for oversized baggage, that term is used quite a lot when in reality people don't realise just how generous the dimensions are. Chances are it wasn't oversized and despite there being a huge emphasis on OTP (On Time Performance) crew don't think twice when bags need to be placed in the hold.