Wonky horizons and verticals are quite common and most post processing software will have easy tools to correct this. As for grid markings on the camera, these tend to be visible on the rear LCD screens which is not something I use very often as I much prefer an optical viewfinder.
My Canon dSRL has a built in 'spirit level' which can be displayed on the rear screen. Not only will it indicate if the camera is level but also indicates the pitch i.e lens pointing down or up. I only ever use this if the camera is on my tripod.
Exposure of some scenes can be tricky with digital just as it was with film. In built exposure 'meters' do a fairly good job. Some scenes however can have a wide tonal range which the metering sort of averages out and often the shadows might be too dark or the highlights too light. As Woodentop refered to - landscapes can have bright skies and dark forgrounds, so how do you get the best exposure.
Years ago when working with film I would Dodge and Burn a print being exposed in the enlarger. In other words blocking off areas that didn't need as much exposure while allowing the rest to have more. In the digital world most post processing software has similar techniques but its always better to get the best you can when actually taking the shot. If the detail in highlights is burnt out you'll never get it back.
I used to use Graduated Filters. A piece of glass/plastic that is opaque but in varying degrees along its length. Imagine smoked glass which is dense at one end and clear at the other. This fitted onto a holder on the front of the camera and you could position it accordingly so that skies for instance would have the denser area and the foreground the clearer.
Its a lot of faffing about but it worked really well but not much use when faced with a charging Rhino.
The digital world software also has the same feature which allows you to recreated graduated exposure across an image. Not only that it can adjust colour, contrast, saturation, sharpness and so on. It works well with RAW images but can be limited with Jpegs.
As Woodentop also refered to you can also use a technique crudely described as 'expose and recompose'. This is where you point the camera at a specific area you want the exposure to be calculated from and then fix this in the camera. Either with an Exposure Lock button or by half pressing the shutter - then moving the camera back to frame the image how you want it.
Half pressing the shutter button is easy but you have to remember it also locks in the focus, which using the AE Lock button doesn't.
Of course the big thing that gets refered to especially with Smartphones is HDR which is where the camera attempts to capture a wider tonal range by taking multiple shots at different exposures and blending them together. I've tried this with my dSLR but never been happy with the results.