I'm A Banana
Potassium-Rich
Well, lets start with you, first. 
Rest assured, you're not a bad DM.
Honestly, I think for long-term D&D players, it's a bit of a psychological thing. Maybe even a left brain/right brain thing. Some people really need the concrete representation of the field, others really chafe under that requirement. Most people probably ping somewhere in the middle, but folks like me just can't ever really enjoy minis-on-a-grid play. And folks like you maybe just can't ever really enjoy all-in-your-mind play. It's not something a logical argument can affect, it's just a feature of how our imaginations work differently. Maybe I'm more comfortable with abstract thought, maybe you're more comfortable with concrete facts. D&D should probably be able to do both.
So you're not a bad DM. Your just a DM who likes the grid. Nothin' wrong with that!
Now, onto the off-grid playstle:
The thing that makes theater-of-the-mind play doable and fun is one simple mantra:
Don't sweat the small stuff.
The exact position of each creature doesn't matter. Exactly how many orcs the mage catches in the burning hands doesn't matter. You make a call that makes sense, and you go with it. You don't worry about the micro-balance of each use of each power and ability. You think about the scene cinematically, you make a call, and you run with it.
Those who enjoy this style don't see the grid as freeing, they see it as confining. It makes them think about little things like the position of each individual orc. Things that, in their mind, don't matter as much. They don't want to waste energy thinking about these little fiddly details. The plastic and the whiteboad and the grid and the measuring tape are obstacles to their imagination, not enablers.
Sometimes these things do matter, even to people who might generally prefer a more abstract system, but not all the time, in every fight. I'm probably a bit of an extreme: I don't think these things ever matter enough to justify the time, cost, and effort of breaking out plastic fobs. But I'm probably at one end of a bell curve.
Maybe you're closer to the other end! 4e had the problem that people closer to my end of the bell curve always. had. to. use. minis. And that's a problem that's just as bad as a system that NEVER lets you use minis.
It seems like a lot of people used minis in their playtest just fine. You should probably go with that! 5e doesn't seem to MAKE you do TotM-style combat.

whearp said:I ask because I ran a playtest over the weekend and found that I absolutely hated the combat.... Am I a poor DM for preferring clear rules and aids for running a game?
Rest assured, you're not a bad DM.
Honestly, I think for long-term D&D players, it's a bit of a psychological thing. Maybe even a left brain/right brain thing. Some people really need the concrete representation of the field, others really chafe under that requirement. Most people probably ping somewhere in the middle, but folks like me just can't ever really enjoy minis-on-a-grid play. And folks like you maybe just can't ever really enjoy all-in-your-mind play. It's not something a logical argument can affect, it's just a feature of how our imaginations work differently. Maybe I'm more comfortable with abstract thought, maybe you're more comfortable with concrete facts. D&D should probably be able to do both.
So you're not a bad DM. Your just a DM who likes the grid. Nothin' wrong with that!

Now, onto the off-grid playstle:
whearp said:I don't see the appeal of keeping track of sometimes dozens of creatures in my head... especially when the monsters are so terribly bland to begin with. It took all my attention and faculties to just keep track of the basics of what was going on, let alone breathe any kind of flavor or life into the encounters....What's so bad about the grid? Why not use the tools we have to allow freedom to focus on other aspects of play?
The thing that makes theater-of-the-mind play doable and fun is one simple mantra:
Don't sweat the small stuff.
The exact position of each creature doesn't matter. Exactly how many orcs the mage catches in the burning hands doesn't matter. You make a call that makes sense, and you go with it. You don't worry about the micro-balance of each use of each power and ability. You think about the scene cinematically, you make a call, and you run with it.
Those who enjoy this style don't see the grid as freeing, they see it as confining. It makes them think about little things like the position of each individual orc. Things that, in their mind, don't matter as much. They don't want to waste energy thinking about these little fiddly details. The plastic and the whiteboad and the grid and the measuring tape are obstacles to their imagination, not enablers.
Sometimes these things do matter, even to people who might generally prefer a more abstract system, but not all the time, in every fight. I'm probably a bit of an extreme: I don't think these things ever matter enough to justify the time, cost, and effort of breaking out plastic fobs. But I'm probably at one end of a bell curve.

Maybe you're closer to the other end! 4e had the problem that people closer to my end of the bell curve always. had. to. use. minis. And that's a problem that's just as bad as a system that NEVER lets you use minis.
It seems like a lot of people used minis in their playtest just fine. You should probably go with that! 5e doesn't seem to MAKE you do TotM-style combat.