What features should I look for in a fireplace grill?
Your primary features should be:


Safe design. This means the unit should remain steady when engaged with the fireplace.
The ability to easily add and remove food from the cooking surface.
The ability to easily move the cooking surface closer to and further from the fuel source.


These are paramount. Your first priority is to do no harm, so you want a stable grill built for the purpose of hearth cooking that won't fall out of the fireplace and set your apartment aflame. The ease of use is also important (and is probably a contributor to overall safety), as you want to be able to control your cooking conditions without much complication.

As a secondary concern, I would want something that either had a rotisserie option built-in, or was rotisserie-capable. Also, I'd want maintenance to be simple. The default thought might be to go for cast iron, but you should consider that it will likely be facing temperatures that will kill the seasoning, and reseasoning could prove to be difficult. So it may be worthwhile to do some research on the best material to use for the long term.