I think this depends on the individual horse, and the individual RIDING the horse. Horses- my horses, anyway- love to be verbally praised. If I advance them slowly, so that if it seems as if what they're doing isn't hard, they love what they're doing. They love it even more if they're praised when doing something correctly. If I try to force something on them without waiting for them to be ready for it (physically and mentally), they get confused, frustrated, and eventually a bit angry. So I think that the training involved deserves some recognition.
Of course, every horse is different. They all have different personalities, and therefore different work ethics. For instance, my gelding LOVES to work, but you have to keep him interested. He lives in with the cattle at home, and chases them around for fun. He even "races" cars along the fence by the road. Mentally, he's like a friggin' ping pong ball in perpetual motion! He never stops! If he gets bored with what you're doing, he changes it up a bit. Thus, I always have to be one step ahead of him, making him think and keeping him guessing, otherwise he does whatever he wants to do. He's probably the type of horse who would be just fine with advancing more quickly, as he just loves trying to figure out what you want. He doesn't seem to be the type to get angry when he can't figure it out; he just keeps flipping through the cards until you tell him he's done the correct thing. I still take it slow though, as he's not physically ready for the majority of those tasks yet.
My mare is the exact opposite. She hates working, and is glad to be a robot-horse. She'll go in circles all day long, and let her mind wander. She's excellent to learn how to ride on, but she seems to like zoning out. When you ask her to pay attention and work, you get a bit of attitude. Get her out on a trail, however, and she's a different horse. She loves working then, as she's too busy looking at her surroundings to realize that she's been going at it for hours.
My other mare is young, and had more behavioral issues before I had someone train her to drive. I always got the impression that she was just bored, and wanted something to do. Groundwork only got me so far for so long. Teaching her to drive (because she's not old enough to back yet) has been a great idea, as she's really matured mentally. I've had this mare long enough to know that if she doesn't want to do something, you're going to have a heck of a time making her do it. But she truly enjoys driving, and hasn't put up any fight whatsoever so far. She's always relaxed and very willing to work.
Each individual horse is different. And they're different on different days. You can't tell me that some horses don't have their "off" days and "on" days!