Dim light has always been a problem for phase-detection and contrast-based AF systems, and at first, phase-detection handled it better. Recent contrast-based systems have narrowed the gap.
Many mirrorless cameras offer touch-screen focus control. Just touch the point in the image on the LCD monitor where you want the camera to focus, and it will focus there. This can be handy for video, as well as odd-angle still photography.
Many find mirrorless cameras easier to focus manually than DSLRs because the mirrorless models provide magnified viewing and focus peaking (which highlights the focused areas in color), while DSLRs have focusing screens designed for AF, not for manual focusing. On the other hand, with most mirrorless cameras, manual focusing is electronic, not mechanical. When you turn the focusing ring or other focusing control, it activates a motor that moves the lens; this eliminates the direct feel and precision of mechanical manual focusing.
HD VideoMirrorless cameras actually have better video capabilities than most DSLRs because their AF systems are faster than typical DSLRs in Live View/Video mode. Of course, the camera's built-in microphone will pick up the sound of the AF motor, so it's wise to use an external mic or focus manually, or both. Many mirrorless cameras offer touch-screen AF, where you can change focus to any point in the scene just by touching it on the LCD monitor; this can be very useful for racking focus in a scene.
All current mirrorless cameras can shoot 1080 full HD video. Panasonic's Lumix DMC-GH3, GX7 and G6 and Sony's NEX-7, NEX-6 and NEX-5T can do it at 60p, Olympus' OM-D E-M5 and Sony's new ILC-A3000, at 60i; other Olympus models and Samsung's NX20 record at 30p, and Fujifilm's X-Pro1 and X-E1, at 24p. The 60 fps cameras can also shoot at slower rates. And like DSLRs, mirrorless cameras can shoot video at lower resolutions (720p, VGA), too.
It's hard to shoot steady video holding the camera out at arm's length, so you definitely want an eye-level finder for handheld video work. Steadiest results will come from using a tripod, but that negates one of the mirrorless camera's main advantages over a DSLR: compact size.
EVF Vs. OVF
DSLRs (except Sony's SLT fixed-mirror models) have eye-level optical viewfinders that provide clear through-the-lens viewing and are familiar to veteran photographers. Mirrorless cameras, having no SLR mirror, use electronic viewfinders for eye-level viewing (about half of today's mirrorless models either have built-in EVFs or have one available as an accessory). Of course, DSLRs and mirrorless cameras have large external LCD monitors, as well; the ones that tilt and swivel are especially useful for odd-angle shooting.
Early electronic viewfinders weren't very good, but today's better ones are very good—better than the pentamirror optical finders in entry-level DSLRs. EVFs can display lots of data, including live histograms and a preview of white balance and other effects. But the fact that they don't provide a continuous view, but rather a high-frame-rate video one, makes them less effective than DSLR optical finders for action and panning shots. But as EVF technology continues to evolve, we expect that electronic viewfinders eventually will replace optical finders on most cameras.