Just to add my experiences to the thread - I went from full frame Canon to M4/3rds (olympus omd em1 plus the pro lenses) then back to full frame with a Nikon D750.
I loved everything about the Olympus but it just couldn't do fast focus and low light, and the smaller sensor is a big factor too...
So, be careful before you buy and make sure that the compromises are not too big...
haha, that was my route - 5D3 (too big - left at home) > OM1 (DoF not shallow enough) > D750 (still a bit big but better than Canon) > XT-20 (so far just right) - missed a few bodies but that was the general progression - reminds me of Goldilocks and the three bears
@Tamar Photography If weight reduction is your main concern then there are a couple of ways to deal with this:
1) Mirrorless - by that I mean M43 or Fuji, I don't see much weight gain in full frame mirrorless as the body may be lighter but the lenses generally aren't.
2) Go prime and carry less - primes generally are lighter and wider, which depending on what you and how you shoot could be a boon - the only problem is a lack of flexibility and don;t get conned by the zoom with your feet lobby - compression comes into effect and the image does change.
3) Buy a good shoulder strap - personally for the heavier stuff (and until very recently the lighter too) I used a Black Rapid Sport - that took the weight off my neck or wrist and distributed the weight
4) Get a Sherpa
Number three is the cheapest and probably a great place to start as thers so little outlay - I used to use the D750 + Tamron 150-600 handheld and when not in actual use it was much more manageable.
On mirrorless Fuji was my choice in the end - part of it is image quality, but another is the fun of having the dials and (SO aside as I've the XT-20, not the XT-2 anymore) you can just look at it and know what you're shooting at even if its off.
On the battery front - yes they do eat batteries, this can be reduced considerably - personally I set the to EVF + Eye - that setting means that it just uses the EVF and only turns it on when I put it to my eye, so generally get a day to a day and a half out of the battery, but they're not expensive - look to the ExPro ones - but the key here is to always carry three. Sure the D750 would last me a week, but there has to be some drawback