Could placing subway stations on a community lot be any problem? I use a lot of these, because they are more space efficient than placing small, dedicated subway stations. My sims can't enter the venue building when it is closed, but that's okay and maybe I let them tag the wall instead...

Subway stations are okay. They are usually outside the venue building and therefore not very much affected by closing times and only your active Sims use it. Other rabbitholes are more problematic because the game pushes inactives to visit them at all times.

Even past 4 in the night? I might be wrong but I recall the game having an aversion to Sims being away from home after 4 and untill.....7?

It depends on their work hours. Sims in the criminal career or Sims in city worlds will visit the hospital in the middle of the night occasionally.

Some good advice here. Also don’t be afraid to use lots of Arsil NPC markers.

So why would there be a problem to place any other type of rabbit hole outside? I thought those that have the opening and closing hours kick sims out of a visible building and escorts them in? Though If it that's not the case, can I add bistro (which works from 11 AM - 11 PM) to a particularly late night assignment club that runs at the evening?

Although you already posted that link to the mod, but thx anyway. I'll try it out.

Maybe we should ask beforehand how many Sims you consider to be enough to call a lot "populated"? What you did to the centre of Sunset Valley makes me cringe a bit, to be honest.

First of all, placing the criminal, school, police, science and spa rabbitholes on the same lot won't help you increase your visitor count at all, because these rabbitholes don't attract any visitors. In a properly working game Sims will only be pushed to these for their work and then be pushed to go home immediately after the shift has ended. They won't stay.
Secondly, unless you use Overwatch, combo-rabbitholes (in your case the city hall-police-military, bistro-business and Business-spa (?)) won't do a thing, because they don't attract visitors. Overwatch fixes that.
Am I getting this right that these rabbitholes are ALL on a lot that is now tagged as a vampire lounge? May I ask what mod(s) you use for rabbithole tuning? Because the blue lounge map tag shows up in your map view alongside the red rabbithole tags, something that is not default behaviour.

Are you really dead-set on using all Late Night bars and clubs? Because as I said in a previous post, in a non-city world they don't pull that many Sims in the first place. The tuning file calls that "intensity". The intensity of a vampire lounge is 50. Now compare this to the intensity of the UL arcade, which is 250, and you can see that an arcade is 5 times more attractive than a lounge, while 50 is on the same level with a beach or fishing spot. Bars score even lower than the lounges (40). You'll have more visitors in a Salon than in a bar, actually (Salon is 75).

In my Sunset Valley I have exactly 1 lounge and it is rather well-visited because it only has to compete with 1 hangout and 1 arcade.

Anything that attracts sims (to a club/lounge/bar/hangout). I learned that they behave differently from other assignments, or shall I say the way it attracts sims.

1. I see.
2. I have overwatch.
3. This was only during when I had just error trap and overwatch.

What about having just a lounge, a bar and a club with only one out of their three types? Would that be in any way of practical? Or should I just go with one of those three (lounge/bar/club).

Depends on the size, of course. A bar will look full with 10, a cinema or a conference center will need at least 30 to get there.

Doesn't something define a world as a town or a city? Does such classification affect the game? Most importantly, how do I switch a town into a city?

All of these questions are answered here:
http://modthesims.info/showthread.php?t=424374
The tutorial is linked in the first post.

Thank you. That would come in handy. Though I wonder If it would be possible to do that to a save file/world.

That's my experience, too. Among the three lounge types "exclusive" is the most popular and "vampire" the least. Similar with the other venue types: local watering hole seems to be the most attractive and dive bar the least; and disco club the best with pool club at the bottom.

That is on the condition that none of these have any bouncer ropes; those tank the rating of every venue.

With regard to the lounges one of my theories is that it may be tied to the wealth of the inhabitants, because I see the rich ones there more often than the poor ones and as any world will become richer over time the exclusive lounge becomes more and more attractive. In my current game, which is still fresh, the fusion lounge fares better than the exclusive one.

Yes, that factors in, too, as defined by the venues.xml.
Apart from the criminals Sims I usually find in dive bars are either evil, kleptomaniacs, daredevils. Tiara Angelista, a 2-star celebrity actress from Bridgeport, can often be found there instead of the upmarket lounges, because she's a kleptomaniac.

How much traits influence behaviour I can currently observe with a Sim that I placed in a world but do not actively play. It's a rich werewolf adult, retired master thief and with the Loves The Outdoors, Great Kisser, Snob, Childish, Athletic traits. Whenever I check I usually find him in the gym, the park with the playground equipment or the nectary (a favourite with snobs) and as he often runs around in his formal wear he must be hitting the lounges regularly, too. I've never seen him yet in the dive bar, or any other bar, and only saw him once in a club.

Yes, you can do it to any world- the tutorial I linked to shows how.

If you want to make a world from a save file you need to follow this tutorial:
http://simfileshare.net/download/385218/

Sorry to chime in here but which rabbitholes do attract visitors?

executive?

Each Late Night lot do attract different types of sims and by their sub-type also. For example, Fusion lounges attract excitable and party animals, while repel Bookworms, Cowards, Loners or Shy and Neurotic. Different story with Exclusive lounge: + Star Quality, Snobs, Schmoozers, Dramatic, Charismatic, and Ambitious. - Evil, Frugal, Loners, Neurotic, Over-Emotional, or Slobs. Same applies to clubs and bars.

The bookstore and grocery stores, the diner and bistro, hospital, theatre, stadium, and city hall. Sims also use the arboretum and the vault of antiquity but I have never seen them visit a lot just for these.

Rabbitholes that won't attract visitors are the military base, the criminal hideout, police station, spa, science lab, the mausoleum, the film studio and all combo-rabbitholes (unless you use the mod).