The things trainees bring in with them can have profound effects on whether the training works.  These include things like cognitive ability (you design training differently for brighter trainees than you do for those who are less mentally adept), motivation (there is often more work getting trainees who are unmotivated to learn complex material), previous knowledge (do you need to cover basic material, or can you assume some level of knowledge on the part of trainees?), and expectations (what do they want to get out of training?).  In a lot of cases, you will want to do some survey work before you begin designing the training program, so you know what youre dealing with.  This is dealt with more in the Needs Assessment phase.

The other thing to be aware of is the extent to which the environment (in particular, the organizations  management) supports the training program.  If there is little support for training, this is something the employees will note, and it will cause them to take the training less seriously, and will ultimately make them less likely to use the material back on the job, since their managers wont encourage them to do so.  Another way to think of this is in terms of reinforcement if going to training is never reinforced, we know that the behavior is not likely to continue.

How the training gets framed by the company is also important.  If the company refers to training as basic, or remedial, that has the implication that individuals who go through it are less intelligent, or generally less sophisticated.  If the company refers to training as advanced, this may lead people to be more motivated, since difficult goals tend to be much more motivating to individuals in organizational contexts, and to lead to higher performance.