Vampire: The Masquerade—Bloodlines was blatantly unfinished when it came out in 2004. Some of the staff at Troika Games scrambled to get a patch together as the studio was closing, but even after that there was still work to be done.
It's janky, but worth playing today, especially in the wake of the Bloodlines 2 announcement. If you're revisiting Bloodlines or playing it for the first time, here are some tips that'll help you get the best experience.
Download the Unofficial Patch
A modder named Werner Spahl, aka Wesp5, took on the work left unfinished by the developer, and his Unofficial Patch is an essential fix. Now up to version 10.3, it has a changelog longer than some novels.
Downloading the Unofficial Patch from ModDB is an essential first step if you want to play Bloodlines today.
Choose a patch version
The second step is deciding between installing the Unofficial Patch's basic version or the plus patch, which restores cut content. (If you got Bloodlines from GOG it will come with the basic version, but without the plus option, so you should still download it from ModDB.)
Some of the things the plus patch restores are questionable, I'll admit. It puts a cop in the tutorial you can talk your way past to learn how the dialogue skills work, and his voice acting is pretty rough. There's an entire sidequest that ends at the library which "never even made it to alpha" according to Bloodlines writer Brian Mitsoda and had to be created essentially whole cloth, with writing that doesn't match the rest of the game. The existing sidequest where a character trades collectibles for sexy posters of the game's lady NPCs—even for a game about vampires, Bloodlines can be pretty thirsty—has been expanded and made even tackier.
And yet, I recommend the plus patch anyway. Some of its additions are rough around the edges, sure, but so is everything about Bloodlines. And among the changes are optional shortcuts that let you bypass some of the late-game combat dungeons that are its weakest points, and a sewer rat who is also a cab driver (though it only appears if you play a Malkavian). It's worth it.
Use the console when needed
It's also worth knowing that Bloodlines has a console you can open by pressing ~. Useful commands are "noclip" for when you get stuck in a door (it happens), "god" for godmode (enter "god" again to turn it off), and "giftxp n" (where n replaces your current unspent xp total). You may not want or need console commands to finish Bloodlines, but there they are just in case.
As for actual full-blown mods rather than patches, there are plenty, but none are the kind of thing you want on your first playthrough. In fact, given how different Bloodlines can be depending which clan you choose, it's worth a second run with the same setup before you even think about tinkering.
Pick a good first-timer clan
Though it's not up-front about this, two of the vampire clans are designed for a second playthrough. The Nosferatu appear so blatantly undead they can't show themselves in public, and you'll spend most of the game travelling via sewer if you play one. They're your hard-mode challenge run choice. The Malkavians have a limited ability to see the future, which means they have a lot of bespoke dialogue options that spoil twists because they know them already. Also, since those insights into the future drive Malkavians mad, playing one means hearing voices and having hallucinations. They're your new-game-plus clan, highly recommended to shake up your second go around.
Any of the other clans are fine for a first-timer. The rebel punk Brujah and animalistic shapeshifting Gangrel are good combat options, the blood magic specialists of the Tremere have the strongest powers, and the artsy bohemian Toreador and power-dressing corporate Ventrue are best at talking their way out of problems. Bloodlines is the kind of game where you can solve things multiple ways, so choose a clan that reflects your preferred play style.
Play it like an immersive sim
Bloodlines is a vent-crawler, not a full on Deus Ex-style immersive sim, but still the kind of RPG where learning how to pick locks and hack computers is worthwhile. The Bloodbuff power can make you temporarily better at lockpicking and Auspex at hacking, if you haven't got quite enough points in the relevant skills. With a high enough score in Persuasion you can often blue-text your way out of trouble, and stealth is a valid option too.
You'll want to explore as well. Sidequests are easy to miss if, for instance, you don't stumble into the abandoned hospital in the Downtown hub, or if you don't hang out in nightclubs. There's a weapons dealer in each hub, often someone who needs to be persuaded to do business, and it's worth tracking them down. You'll be stalking the alleys to keep your blood topped up anyway, so poke around any suspicious places you find while sneaking about.
Bloodlines is not always great at being a stealth game, however. Your score in Sneaking is more important than sightlines, and a lot of places that appear well-lit have guards who act like they're in deepest shadow. Maybe those tunnels look so bright because vampires can see in the dark? Yeah, let's go with that.
Learn to fight
While stealth and charm can get you through most of the early game, eventually you'll need to kill a whole mess of people. Chat with Nines Rodriguez, who hangs out at the Last Round, and he'll put it plainly, saying "a speech ain't gonna save your ass when you're staring down the barrel of a shotgun." (He also helps improve your Brawl and later your Melee skill if you ask for some pointers.)
Bloodlines hands out plenty of experience points, especially if you're tracking down all those sidequests, and you'll need to start jamming them into combat abilities eventually. Melee will deal with the first batch of fights (the fire axe is an excellent weapon at this point), but later it'll be worth going for Ranged Combat as well. The plus patch does add shortcuts that'll let you skip past the most boring combats—there's a computer you can hack to unlock a door to bypass most of the sewers, and if you make friends with NPCs Yukie and Chunk then max-out Persuasion they'll offer shortcuts too—but boss fights can't be avoided and they really are some bullshit.
That's because with the right weapons they're trivial, and without them they're ridiculous slogs. They typically come at the end of areas which you don't have any way of backing out of to quickly duck to the shops too, so check in with weapon vendors frequently. Their stock updates after most of the main quest steps. Buy new armor, grab a flamethrower when it appears for sale and, if you haven't looted one from a hunter, get a machine gun. They'll be worth it when you fight the bosses who mutate into warforms like something out of a JRPG.
Bloodlines can be a very 2004 game in some ways, and it still crashed to desktop a couple of times on my last playthrough (F9 is quicksave and you'll need it), but it's worth playing. Its combination of a contemporary setting, so-goth-it-shits-bats atmosphere, and strong character writing make it unique. Until the sequel comes out, there's nothing else like it.