Just like with engine oils, UTF's and gear oils there are many properties that come into play to make a superior lubricant. With grease some of the key features to look for are load capacity, water resistance, cohesive/adhesive properties, compatibility, temperature range based on application and oil seperation characteristics.
Let's start with water resistance. It's fairly straight forward. The greater the resistance to water the better. If it resist water it will repell it forming a water resistant coating on lubricated parts preventing rust and corrosion. The greater it's water resistance the better it can maintain it's load rating when contaminated with water. The best greases can maintain their load rating even in the presence of 20% water. A demonstration I do for water resistance is done on my car or truck side window. I take the competitions grease and smear a little on it (my window that is), take a handfull of paper towels, soak them with water and spray water on the window while I try to rub it off. Eventually I end up wiping off all the competitions grease. Then I stick some of my grease on the window and do the same. All I end up doing is spreading the grease around, you just can't get it off with water and paper towels. You have to use solvent to remove it.
Cohesive/Adhesive properties. This is the "stickyness" of the grease. The cohesive properties is what makes it stringy. The more cohesive it is the more resistant it is to being pulled apart. In other words, it doesn't want to let go of itself. The adhesive properties are how well it sticks to other things, your tractor and you (and much to your wifes dislike your cloths).
There is a fine line in how cohesive and adhesive a grease can be. I have seen greases that were to cohesive that they actually balled up and fell off on rotating shafts. A properly formulated grease should be just adhesive and cohesive enough that (lets say on a pillow block bearing and shaft) it should be able to form a barrier against contamination/dirt yet resist balling up and falling off. Basically it should leave a ring on the non rotating part of the bearing and the same on the shaft. It more or less forms it's own seal against dirt entry. Cohesive/adhesive greases will also greatly help to reduce grease consumption and actually help to reduce wear. If the grease resist being pushed out, slung off or washed off and stay's in place the next time you have to grease you don't have to use as much. A good grease may cost more but if you only use 1/2 as much where's the savings in the cheap stuff. Also if it is staying where it should, the lubricated part, you reduce wear. When the grease is not cohesive and adhesive enough and is forced out under load there is nothing left to protect the equipment. Where do you want your grease? On the ground or where it should be, in the lubricated part doing it's job.
Compatibility isn't a huge problem but something to watch for. Some grease thickner types are not compatible and when two non compatible greases are mixed the end result is a grease mix with greatly reduced lubricating properties and also tends to become thinner. If switching grease thickner types just lubricate a little more frequently until you get the majority of the old grease purged out.
Temperature range, it's a no brainer. If you are lubricating bearings in a furnace you want a ultra high temperature grease that never melts. In most tractor applications you don't need a ultra high temperature range. Something around a 300 deg. F. dropping point is typically fine for a tractor. You need to watch temperature ranges when you start to get into things like electric motor bearings and disc brake wheel bearings etc...
Oil separation is just what it sounds like, a greases characteristic to bleed/separate oil from the thickener. Ever opened a pail or tube of grease that has been setting for any length of time and seen oil? Picked up your grease gun and seen oil run out the end? That's oil separation. A good grease will not bleed it's oil out of the thickener until it is actually in use where it bleeds out a controlled amount to do it's job, lubricate and can reabsorb any oil that does bleed out. I have pails of grease at my place that have been setting for over 12 years and show no signs of oil separation or cracking/shrinking of the grease. A grease with excellent oil separation properties should in all honesty never go bad setting on the shelf, even 30 years later.
Load rating is a greases ability to protect from wear under extreme pressure/load. The greater it's load rating the more protection it provides at higher loads. If you are lubricating items like FEL's and BH's you want the highest load rating you can get since they place enormous amounts of load on the pins and bushings.
Another option you can get in many greases is the addition of Molybdenum Disulfide (Moly). In applications like FEL's and BH's it is a excellent wear preventative. In these applications even the best grease on the planet will at some point be pushed out. The Moly forms a solid film on parts and acts as a last defense against wear. It is also great at reducing frictional heat. John Deere actually spec's a 3% Moly grease on their excavators. A good Moly grease will have 5% Moly by volume.
I'm sure I forgot a couple of things but its a start.
