One of the more overlooked aspects of gameplay in Diablo 2 and 3 is the follower system. As small as it seems to be, having a follower fight alongside you is always a good idea, even if the follower you have is obviously weaker than your character. Just having a follower to buffer damage from enemies is an advantage against enemy mobs in a singleplayer game.
First off, the similarities between the two games with followers is that they can be equipped items and have abilities for damaging and/or slowing down enemies. There are also different kinds of followers, although there's more variety in Diablo 2 than in 3. Perhaps Blizzard can add another follower in Diablo 3 with a future content update to rectify that, but it's not that likely at the moment.
However, there are tremendous differences in how both games handle followers. For instance, you hire followers from an NPC in each act of Diablo 2, while follower NPCs willingly join you during the storyline in Diablo 3. While each Diablo 2 follower has a spell or special ability (excluding the ones from Act 1), the Diablo 3 followers have a mini skill tree of sorts with two skills to choose from in each tier to improve the utility of the chosen follower.
Beyond that, the unfortunate thing is that Blizzard has not done anything beyond this for followers in the two games. There's some potential to be had in the system, but even Diablo 3 doesn't get a lot of content updates these days, so it's not likely that anything more can be done for followers in that game beyond what's already there.
That's too bad as the commonly held belief among players is that the Templar is the default choice, with the Enchantress as a more novel but still useful option and the Scoundrel is not that useful at all. If only Blizzard can do things such as buff the Scoundrel to make him more useful, then followers can be made better in that game.
With that said, perhaps Diablo 2's follower system seems much better due to being able to better strengthen the follower with equipment that boosts stats. While the same can be done in Diablo 3 to an extent, they're still seem ineffectual compared to Diablo 2 followers.
You can balance their Diablo 2 items in order to enhance their survivability and helpfulness in battle. With that said, you'd still want to reserve the best gear you have for your own character, but whatever your character outgrows can be kept for consideration for outfitting your follower. A well-equipped follower can actually do well in making fights easier in that game.
But other than that, there isn't much else to it. It would be nice if you can change their abilities and control how they behave, but perhaps a part of the essence of followers is that they're mostly autonomous and you have to pick one that you think is best for you.