I like the basic design of garrison followers. I liked recruiting them, leveling them up, and improving their gear. However, as with everything else, I think followers had some issues.
Mission planning was just too complicated and tedious. There's a reason everyone used Master Plan, and that there was such an outcry when it stopped working with the 6.2 patch. Maximizing rewards by assigning followers to missions was an easy thing to automate, so it was automated.
I think Blizzard would have done better to make this simpler. For example, one thing they could have done is limit the number of available mission slots to match the number of characters you have. Thus you can't get 100% on all missions but must choose the missions you most want to win.
Or alternatively, perhaps at the beginning of the week you organize your followers into parties, with one tank, one healer, and three DPS. Then you are presented with 4 missions and you assign a party to each mission. By reducing the number of possible options, you greatly reduce the solution space, and make it much easier to do follower missions by hand.
I don't think that timed missions were a good fit for MMOs. Timed missions might be good for mobile games, because your phone is always on you. But I don't think they match the rhythm of an MMO.
I think missions should have been more like dailies. They all complete at the same time each night. Then you can assign new missions sometime during the next day. I think that daily or weekly reset is a more natural fit for an MMO.
Garrisons had a neat element where your followers would hang around your garrison and interact with you and other NPCs in small ways. Unfortunately, 99% of the time followers were out on a mission, so you rarely saw this element.
I think Blizzard should have just left your followers hanging around, even if they were technically assigned to a mission. Or possibly have the non-active followers hang around. (I don't actually know if this happens or not, I only have one inactive follower.)
I like the follower bodyguard. I run with the draenei paladin tank. If I'm in a quest with Yrel, that's three paladins running around, delivering holy justice to the orcs. Good times.
However, the fact that you can click on the follower to bring up dialogue drives me nuts. It happens when I try to loot, and it happens an awful lot. Regular hunter and warlock pets don't have this issue, and I really wish Blizzard had thought more about this. The barracks should have just given a "Go Home" ability that you could put on your bars.
Overall, followers were a pretty good idea, and an interesting part of WoD. But the four issues above kept them from being great.