If a friend tells you they just finished a really good workout, what does that mean to you?
What elements must a workout possess to be considered "good?"
If you ask 100 people this question, I'm sure every single one would mention one thing: the workout has to be hard. The harder, the better.
In the eyes of the average person, intensity trumps every other element when it comes to assessing the effectiveness of an exercise session. If the workout leaves you on the floor gasping for air, it was a good workout. If your muscles burn to the point of agonizing pain, it was a good workout. If you went so hard that you can barely lift your arms to drive your car back home, it was a good workout.
If you're a trainer, you can mess up every other component of a client's exercise program, but if you nail the intensity, the client will thank you at the end of the session. "You kicked my ass today, coach!"
A good trainer, however, understands that intensity is just one piece of the proverbial training pie.
To answer the question of what constitutes a "good" workout, we must consider the following:
Do the intensity, volume, and exercise selection match the trainee's fitness level?
Does the exercise order make sense from a physiological standpoint? How about the rest intervals?
Is the nature of the workout consistent with the specific goals of the trainee?
If weights are being lifted, are they heavy enough (but not too heavy) to reach that sweet spot required for progress?
Is proper technique being used or is it sacrificed for the sake of doing more work?
These are just a few elements to consider, but you get the point.
When it comes to answering the original question, there's a disconnect between the average person and a fitness professional. The reason this can be problematic is that you can perceive your training program as being amazing when it's only ticking one of many boxes.
For example, I can write you a program that completely annihilates you in 15 minutes. It'll be a circuit involving high-intensity, total-body movements—things like burpees, mountain climbers, jump squats, etc. The rest will be minimal. You'll be out of breath, sweating like crazy, and your muscles will burn.
Is it a good workout?
Well, it's definitely not good for gaining strength or building muscle. You need heavier loads and long rest intervals for that.
It's also not ideal for cardio. A more effective method would be separating your cardio into Zone 2 and HIIT. The first involves a relatively low intensity for a long duration. The latter involves very high-intensity work, followed by adequate rest, for rounds.
Lastly, it's not good for practicing quality movement patterns since technique will go out the window when you're exhausted.
So no, I wouldn't consider it a good workout because it could have been much better with a few simple tweaks.
Another case against overemphasizing intensity is that it's possible to train a muscle group so hard that you're uncomfortably sore for days following the workout. This means you're less likely to get back in the gym for more training. A better approach is to spread the volume of work over several days instead of going so hard just once.
Don't misunderstand the message. Intensity matters; it's just not the only thing that matters. You shouldn't sacrifice other workout elements, such as exercise selection and technique, for the sake of pushing yourself to the max.
One last thing. This is the perspective of someone who looks at exercise through a very technical lens. I believe we should optimize for effectiveness when thinking about training, but there's still a degree of subjectivity to the topic at hand. If you think a workout is good for the sheer reason that you enjoy getting your ass kicked, then don't let me stop you!