I mostly see solar panels on peoples roofs. If space is not a problem, and one had a few acres of land, is it better to place the panels at ground level? I can see the benefits of ground level panels being able to service them, being able to rotate/move them during the year to gain the maximum direct sun exposure, and finally being able to clean them off if needed.

Quite a few panels end up on roofs and I think that is primarily because a lot of people live in built-up areas and just don't have suitable space to put them on the ground.

However, among the people who have a choice, you still see a lot of panels going up on the roof. As someone who thought putting them on the ground might be a good idea, let me mention a few reasons I have decided to go roof-mount.

1. There is a substantial cost to installing suitable ground mounted units, particularly if you want to have something that is adjustable.
2. Building departments seem to have little experience with the ground-mounts and things are more difficult.
3. You generally need longer cable runs with resulting cost and loss of efficiency with the ground-mounts.
4. Ground-mounted units are more susceptible to damage from LOTS of things (baseballs, vehicles, animals, etc.), while roof-mounted units are slightly more at risk from wind.
5. It is easier to wash and adjust ground-mounted units, but that doesn't mean you are prohibited from doing it on roof-mounted units, particularly if you can build to suit with a low-pitch roof or metal roofing.
6. Loss of "yard" space. I have 20 acres, but I still value the yard space near the house. Basically, solar panel mounts would take away from fruit or nut trees or patio or something.
7. View blockage.

I've always liked the in-between solutions such as building a carport or a firewood storage area in which a substantial portion of the structure is devoted to PV panel mounting. Of course, with those you lose some of the adjustability factor.

ICFHybrid has covered the advantages and disadvantages well. By convention, people that live in urban areas tend to put solar panels on the roof. People that live in the country go either way.

I do find it a little effort to clean the snow off my roof-mounted panels. Curiously to me, covering the bottom 25% of a row of panels with snow reduced the power output by 90%. Makes we wonder if the shadowed cells become electrical shorts (they are diodes) for the active cells. This effect come have implications for shadowing in general. I will investigate further.

This effect come have implications for shadowing in general

Exactly. Even a little shading like a vent pipe that shades a small corner of one panel can kill the power from an array.

Some solutions have been found in breaking up arrays into smaller arrays so that the impacted panels drag down less of the total output, or even using "micro inverters" in which each panel has it's own inverter.