I felt the first Bioshock had a few gameplay issues. The low ammo and respawning when you die promoted me playing the game simply with the wrench, because you'll never actually completely die, while saving the limited ammo for tougher battles. But this style ruined the gameplay.
Bioshock 2 doesn't fix this issue, but it does improve it by making your melee weapon - the drill - probably the best weapon in the game. But this does mean most of the other weapons hardly get used. The Rivet Gun and Machine Gun I used sometimes, the Shotgun a bit less, the Spear Gun was far too slow and the Launcher I didn't use a single. I had the most fun with the drill, especially freezing enemies then wacking or drilling away at them.
Sadly the game still uses respawn points everywhere. They're much more common now, so you won't have to trek long distances, but it still means there's very little reason to take on a Big Daddy or Big Sister with anything other than the drill and simply respawning a few times.
In cases where your Eve is about to drop down a syringe - why bother wasting one, just let yourself die and you'll be maxed out again. While the gameplay is very fun, and should be challenging, it's not when you cannot die. Even the end battle was a complete breeze when you have your team mate Eleanor and the ability to quickly replenish all your health, eve and ammo many times over for little money. The game does have some challenging parts though, and that's when you need a Little Sister to extract Adam while you fend off a wave of enemies. This is the only point in the game where you cannot respawn otherwise you have to start the process again. So in these situations you had to go in with full health and possibly use a healthkit, especially in the second half the game.
The upgrades (plasmids and tonics) were all good fun. Tonics simply give you slightly better abilities, but plasmids allow special attacks. The picks of the bunch for me were the decoy, freezing enemies, electrocution and fire. Great fun and probably a bit more refined from the first game.
The design of the first Bioshock was the real highlight, and fortunately Bioshock 2 looks just as good, probably better. The engine is still the same as the first game, the modified Unreal 2.5 Engine. It 2010 when most games now use UE3, Bioshock 2 is in danger of looking slightly dated. Alas, it doesn't one bit. The game still looks fantastic, but perhaps the indoors and often small drawing distances allow the developers to put full effort into the detail. While Bioshock really missed the opportunity to create some scenic underwater views, Bioshock 2 fixes that and even allows you to travel onto the ocean floor a few times. Sure it was extremely linear and short, but the set pieces are much more common now.
The visual effects are also pretty good in Bioshock 2 - water looks particularly impressive especially one level where the antagonist floods the area while you escape. This had to be the most impressive part of the game - it looked fantastic.
The only real problem with Bioshock 2 is that a lot of gamers might be put off by a lack of evolution. The game is basically the same as the first - not really pushing any boundaries but just improving and tweaking probably every area. I think that's why Bioshock 2 hasn't really had the same acclaim and review scores as the first game, but ultimately Bioshock 2 is the better game.