You may feel cheated when I combine these games together into a single review, but when I stopped to think about it, they are actually very similar games. While they may follow very different control schemes, plots, storylines, are set in different fiction genres, and in fact belong to completely different game genres, the two games are actually quite similar in terms of basic thought processes.
You begin each game in a strange environment. People around you seem to have a vague idea of who you are, but you're more or less a stranger to them. However, they still need your help, and you are given a number of quests that you have to complete in order to progress.
As you play through the games, you will encounter various enemies. How you kill them is your own responsibility. The game does not discriminate against the way you choose to inflict damage, but certain enemies will be resistant or weaker to specific types of attacks. Switching methods of attack on the fly is key to defeating waves of enemies without too much headache.
When you kill enemies, they will give you loot. This loot is randomly generated and not fixed to any specific progression. Depending on what kinds of items your kills give you, you may choose to focus on a different method of attack; favor a different type of weapon. The difference between what weaponry enemies give you, what you choose to use, and what weapon is most effective against them is the general balancing act of the gameplay, mostly by choosing the most effective weapon for each situation, based on range, speed, strength, and ease of use. Necessity (or laziness), however, may tempt the player to focus on one or two main modes of attack, and only use others when absolutely necessary (or when bored).
The number of weapons you have access to in Bioshock is much, MUCH smaller than in Diablo, but the overall number of weapon types are more or less equal. Weapons can be infused with elemental properties: ice attacks will freeze enemies and make them easier targets, but fire and electrical attacks are also available. Certain enemies are much more vulnerable to particular elements, while others are resistant or even completely immune.
While Bioshock and Diablo both follow a more or less linear plot (You complete the same quests in more or less the same order in every playthrough) in Bioshock, every weapon and enemy you face appears in generally the same location every time, and in fact every map is identical to every playthrough. Every key item and weapon you find will be in the same location and found at the same point in the plot; in some cases these are even required in order to progress.
Diablo, however, gives you a wide selection of weapons. While any specific aspect of them can be simplified by attack speed, range, and whether it is used in one or both hands, these weapons are replaced with stronger varieties throughout the game, andyour choice of what you use is made more complex with the addition of magical modifiers. Magical weapons can be found that provide any number of bonuses, from increased attack rating, higher health and mana, elemental properties, and much much more.
Unfortunately, The majority of weapons you find will be completely worthless to you. Modifiers such as "low quality" "crude" "damaged" or "cracked" are just as common as an unmodified or "superior" weapon, and these weapons are generally not even worth picking up to sell back in town; they aren't even worth the space in your inventory. And even when you do find a magical item, odds are you won't be interested in the item's particular modifiers. But when you do finally get that perfect item you've been hoping for, it's one of the best endorphin rushes in gaming.
Replay value in Diablo 2 is made much higher than in Bioshock based purely on the sheer number of ways you can play through it. Every class has 30 skills, each of which has 20 levels of effectiveness. Since it's impossible to gain 600 levels, you will have to choose what skills your character will focus on for any individual playthrough. Multiply this by 5 classes (7 in the expansion) and the infinite number of maps that can be randomly generated, and Diablo 2 is a game that anybody can play through multiple times and never have the exact same experience.
But the burning question is: would you want to?
What Bioshock lacks in complexity, it makes up for in focus. While it doesn't do nearly as much as Diablo 2, what it does do is highly polished and enjoyable. It may not be as long, but the fact that it can be played through and experienced much quicker means the entire narrative arc can be enjoyed in a shorter amount of time, and gratification is easier to come across. The game is too short to become monotonous, and too focused to become diluted.
Diablo's enjoyment comes from the progression of your character and growing him/her based on your method of play. You become attached not only to your character, but even your items. You hesitate to throw away that awesome sword you found in act 1, that served you so well for two entire acts and matches your character's skills perfectly, even though it has been rendered obsolete by newer equipment and is taking up too many of your inventory slots. But eventually you bite your lip and sell it to the merchant with a heavy heart, replacing it with something that may serve you better, but has no nostalgic attachment... yet. But perhaps in time you will learn to appreciate this one just as much.
Which is a better game? Who can say. It's all a matter of how you play it.