This guide, while short, will detail the best way to change L4D2 into your perferred experience using workshop content.

Weapon content usually comes in three forms: Sounds, Skins and Animations.

When choosing Sounds, try to match the theme of the weapon and/or the other content you've downloaded. A pistol with custom animations that fires like a big, bulky handgun works best with clunky shot sounds, whereas a smaller, slimmer pistol is better off with quieter noises. (I normally change the noises myself by going into the game's files, but it's easier and faster to choose something you like from the workshop.)
Also, consider how you would like the game to play. If you find a realistic animation or skin you like, then a realistic sound will work brilliantly, but a cartoonish or futuristic one won't.
I could go into detail with melle sounds and such, but they are minor details to a weapon's feel compared to the shot and reload noises. If there are two packs that you like different sounds from, then you may have to edit them together yourself.

Skins are also a major factor in the experience. A Gabe Newell frying pan might seem cool, but it can deduct from the realism. On the other hand, a realistic pump shotgun will make an anime-based zombie-fest less catoony that it could be. If you have a skin you like, then keep it, but don't fill your experience with out-of-place firearms.

Finally, it comes down to Animations, you really need to be going for realism to find these useful. Most, if not all, of the custom animations on the steam workshop are based around real-life firearms or movements, making the game a bit more like a modern shooter than a comedic zombie slaughterhouse. Like skins, if you find a cool animation then go for it, but don't make every gun act like a COD or Battlefield weapon.

While custom survivors and zombies are common on the workshop, remember to keep a theme in mind when choosing your skins. Three milllitary player skins and one stormtrooper skin won't work unless you aren't picky about realism, but even in comedy-based combinations of skins the experience can tither between funny and stupid.

When choosing character models, think about their personalities and abilities. Coach in a One Direction shirt may suit your fancy, but remember that Coach is a Midnight Riders man. If you still think your skin is fine, then nobody's going to stop you from using it, but if you are dedicated to the game feeling a certain way then it may be time to rethink your choices.

Zombies work in basically the same way, although there isn't really the personality factor. Still, consider any skins that don't work with others if you feel like doing so. You may be suprised that one of your skins actually doesn't work with any others. Try not to download skins that give you an edge, such as neon-shirted zombies, as not only will it damage the experience, but it will make the game seem easier, even on harder difficulties.

Character sounds and voices should be easy to compare, as most sound packs are clearly named in ways that show if they are meant for realism or for fun.

The music of L4D2 is good without changing it, but you may grow tired of the same soundtracks eventually.

The idea of choosing music is easier than choosing other content: watch the attached videos or look the song up to see if it's good enough. Certain factors should be considered when going for realism, however...

1) Ending music should be something that makes you feel successful, such as songs containing words suted to winning the campaign.
2) Horde music should be creepy and easly audible at the same time.
3) Safe room music should be like your ending music, but less celebratory.
4) Special infected music should suit the infected: tanks (and possibly witches when startled) need a big, noisy theme to show their presence, while hunters or other stealthy infected need tinkling little tunes to minimise the survivors knowing that they're there.

Following these can make your realistic feel of the game much, much better. For more random or fun-focused gamers, choose whatever songs you want to have!

There isn't as much I can write about this topic as the others, due to most maps being easy to recognise as realistic or unrealistic. If there's a map you aren't sure about, look up walkthroughs, ask people who have completed it or try them out for yourself!

However, be advised that some campaings change skins and add new items. This could complete or destroy your experience, so think twice before commiting to a cool-looking set of maps without looking at the descriptions.

One thing to remember is that you don't have to play every map you download. If an unfitting campaign pack has a fitting survival level in it, then just play the maps that work with the feel you want.

Remember that you can create and modify maps in the L4D2 Authouring Tools, so if you find a map that isn't as dark/light/spooky/random/hard enough, you can change the way it plays!

I hope this guide helped you choose content that you enjoy!
If there's anything I haven't covered, just comment below and I'll try to add it.