Articulation
Posability is the ability of a toy to be posed and moved. It is highly dependent on the toy's acticulation, or the number, position, and type of its joints.
The more joints on a figure (in either form), the more articulated it is. This generally includes the shoulders, elbows, hips and knees, and sometimes even wrists, ankles, and neck. Posability in Transformers ranges from toys like the Spy Changers, who are limited to rotating their arms up and down (shoulder swivel articulation only), to the non-transforming Revoltech figures, whose entire selling point is numerous points of highly-posable articulation. Possibly the only Transformer toys that have no real articulation, but can still be said to transform, are Under-3 and, arguably, Freedom Fighter.
Posability is one of many factors that fans weigh when evaluating a toy, and naturally is of subjective value. To some, if a toy has unusually good posability, they will buy it over another figure that looks "better", but can't move as much. Toys with very few points of articulation are often referred to as bricks. Many modern Transformer toys, particularly those from the Beast Era and Robots in Disguise, have high posability thanks to the incorporation of many ball joints.
Unfortunately, there is a tradeoff between articulation and production cost -- the more complex the figure, the more parts that must be assembled and therefore produced, and the higher the retail price. For example, Armada Megatron could easily have been given knees, and many kitbashers have taken to altering the figure to be more posable. Some fans claim this is how Hasbro "should" have made the original, but doing so would likely have sent the figure over budget.
Some figures are hampered in how well they can pose by their transformation or altmode. Armada Hot Shot's shoulders, for example, are limited by how he transforms. A different transformation may have allowed his arms to move on an additional axis... however, as noted above, this would have also increased the toy's complexity and cost. Posability can also be limited -- even when the needed articulation is present -- by kibble or other parts blocking a part's motion.
Conversely, sometimes a toy's posability is improved by its transformation. Or, perhaps more accurately, the designers managed to place the transformation-related joints in places that would also serve the robot mode's articulation. The Unicron toy released during Armada is an example of this. His neck, shoulders, arms, wrists, hips, ankles, and even feet all have to be moved to change him from planet to robot and back.
Articulation can have a drawback: If a figure is overloaded with joints, especially in the legs and waist, this can result in the figure having problems supporting its own weight, holding a pose, or even standing instead of toppling over. This is a common complaint with Omega Prime -- he is loaded with exciting and dramatic articulation, but is so top-heavy that it is hard to get him to do anything beyond "standing up straight." The much-loved ball joints are especially susceptible to weight problems, and as such are generally not used on larger figures. A similar level of flexibility can be created with a combination of two swivels (perhaps ratcheted swivels), but this increases the toy's complexity and cost. As a result, many large toys have limited leg articulation.
