Pixel count is an important consideration. Some high end full frame cameras have 16-20mp, which gives them fast frame per second rates >10fps, and excellent ISO/low light performance - qualities important for sports, and other kinds of photography where this is helpful.

People who shoot wildlife and landscape with good lenses often want the highest number of pixels, and though for birds, a high frame rate is helpful, you can get outstanding images with a camera that only does 5 fps. But being able to crop a 36mp or 46mp image without a serious loss of image quality has it's merits.

Then there are the smaller APS-C, M4/3 and smaller sensor cameras. In each of these you will find compromise - the first would be low light performance. The second would be loss of sharpness due to diffraction. The third would be differences in depth of field. As the sensors get smaller you need to use wider lenses for similar angles of view, and that will negatively impact your ability to get really shallow depth of field. The M4/3 cameras have extremely fast lenses available, with maximum apertures between F.95 and F1.2 to mitigate the depth of field issue. On the plus side, M4/3 lenses are typically high acuity, contrasty lenses yielding excellent image quality in a small, lightweight package.

Some cameras, like Pentax and Olympus, offer a "super resolution mode" that shoots multiple images with a very slight sensor shift to enable processing in camera of a super resolution image. A 20mp camera is then capable of a 40 or 80 mp image. This is not magic as it can be done in post processing with images from any camera, but it's nice that it is internal. The downside is that it is useless for images of moving subjects.

If you like the D300, the obvious choice would be the D500. It will feel familiar in your hands, and with a 20 mp sensor, you have a nice balance of speed (10 fps) and good high ISO performance. If you want better quality and more cropping options, a D810 or a D850 would not have many compromises. Canon has similar offerings, being very competitive. I think Canon's lens standards are higher than Nikon's on the whole, but this would be splitting hairs because often the differences exist on paper and not in the field.