In some area, size does matter. The camera bodies are smaller but the biggest difference is the lens size. I can pack one body and three lenses in a small bag that would have held only my 5D Mark II and a 35mm f/1.4.
The client or model is usually less intimidated in front of a small camera. In some types of photography, like street photo, it's useful to be less conspicuous.
Weight is such a big factor, not just for the physical well being but also for mental health. Lugging 40 pounds of gear all day for a wedding leads to being so tired that it's hard to stay concentrated and creative. With mirrorless cameras, you can cut the total weight by two, three or even four times. My back is much happier at the end of the day and my mind finds it easier to be alert to what happens around me.
It makes travelling easier too, and you can save money since you won't get charged extra air fares for being over the allowed weight.
I admit it takes a couple of weeks to get used to the Electronic View Finder (EVF) but I would not go back to an optical one. The only exception to that might be for studio work with flash. With the EVF, what you see is what you get. When you change the exposure compensation, you see the result immediately. You know before you even hit the shutter that you nailed your exposure. You don't have to chimp every time you click the shutter (I call it pre-chimping ;)). It allows you to stay more in touch with your client or model.
4- Live Histogram
Having the histogram displayed on the screen or in the EVF is invaluable. In hard lighting conditions, it helps a lot to set the correct exposure. You can avoid blasting the highlights easily.
5- Image Quality
Two years ago, I was a guy that was convinced that you needed a full frame camera to have good image quality. I was not missing my Canon APS-C years at all. I decided to buy a smaller camera for my personal work and family travels. I was astounded by the image quality of the Olympus OM-D E-M5. That little camera was offering an image that was as good as my Canon 5D Mark II. There were some differences (mainly the depth of field) but overall, the contrast, color reproduction, dynamic range and sharpness were excellent. From that minute, when I checked the photos in Lightroom, I was sold.
6- Focus Peaking
To be honest, when I received my Panasonic GX7, I had no idea what focus peaking was. It is the kind of thing you do not know you need until you use it. I was using manual lenses on my 5D Mark II and I bought a better focusing screen to help me at the time. It was a little bit easier but focus peaking makes manual focusing trivial. It is a function that adds a color overlay on the area of the image that is in focus. You also have the option of enlarging the area of focus when you touch the focusing ring.
The biggest benefit of that is the use of old manual lenses. You can use any lens you can find and it is really easy to focus manually and be sure you are spot on. There are adapters for almost any mounts. That means you can use almost any lens ever made and still be able to focus easily.
It is also invaluable in video. It makes doing rack focusing easy and it helps a lot to track a subject in movement.
6- WiFi Transfer
That is the kind of thing I did not think I would use and now I can't live without it. You should have seen the faces of the artistic director and the client the first time I used WiFi transfer in a shoot. They were both sitting comfortably on the couch in the studio with a tablet in their hands and they were seeing the photos while I was shooting. It was easy to make small adjustments to the setup with their feedback. They loved it.
It is handy for event coverage, like a press conference, because you can transfer the photos as you shoot and it's fast to get them online or to the news agency. When you travel, it makes it easier to update your blog or your social accounts with the nice images you just took.
7- WiFi Control
Controlling your camera from your phone or tablet can be very useful and here are a few examples.
- You put your camera in a place hard to reach, but still retain the control over it. For example, you put it over a basketball net to take very cool shots.
- You use it on a monopod that you hold high in the air, and use your phone to frame and take the shot or even select the point of focus.
- You have the camera on a slider or a steadycam. One person can operate those toys when another one can rack the focus with the phone.
- You want to take a group photo with you in it and don't feel like using the timer and running over to the group.
8- Silent Mode
By Silent Mode, I mean completely silent. This is possible with the use of the electronic shutter. In this mode, the camera does not emit any sound. It is perfect for shooting in a church or during a speech.
9- Touch Screen
That is another thing that I did not think I would be using much. I was proven wrong once again. It is incredibly useful for selecting the focusing area when you are low on the ground or high in the air. You can even set it to focus and take a shot when you touch the screen. I also use the screen as a joystick when I'm looking through the viewfinder and want to change my focus area. It is faster than using the arrows.
10- The Fun Factor
Ok, that one is not only for mirrorless cameras, but when I started using them, I fell back in love with photography. I have not had so much fun using a camera for many years. I can't put the finger on one thing in particular. I think it's probably the combination of many things, but one thing is sure, I take more photos when not "on the job". It's a joy to carry around my GX7 when I go out with my family. Even the kids take photos with it (and they love that). A seven years old trying to hold a 5D Mark II is a scary thing and you might not get well composed and stable shots out of it. Give him an OM-D and you will be surprised what they can get out of it.