Reading the comments here, both the pros and cons of open neighborhood, it appears to me that one issue a lot of people are worried about is technical. Lagging issues in open worlds, or long loading times in a more closed world. I think with sims 5 it would be safe to assume it will run on a more efficient gaming engine and that it will work better than both sims 3 and 4 as technology is so much better today, so for me, those issues are non-existent in a discussion on what I'd prefer.
For me these are my objections to both open and closed world (and why I prefer open neighborhoods).
An open world sounds in many way fantastic, but if it is a big world, your sim still needs to get from point A to B when going somewhere and that would take game time, especially if it's in another part of town all together. I'm not really interested in watching my sim drive (or take a bus/taxi) any more than I am watching a loading screen that takes forever. When going far I want to get there fast, and here I think a quick loading screen will be the faster option.
However, with a closed world we have other issues. For example, right now my sim lives next to her sister who is married to one of her best friends. They both have two children (girls), and get along really well and summer is coming up. Now I see a situation where the four girls run in and out of each other's houses and the sister, her wife and my sim chat over fences and move seamlessly from one household to the next. Only with a one lot system as in sims 4, they can't without loading screens and formal invitations. That breaks immersion.
Another example, I had a sim who loved to party and lived next door to a nightclub. Literary crawling distance home. Going there was one thing - they'd walk outside their door, disappear and show up at the club. Slightly annoying but manageable. Going home however was another matter. Every time my sim should go home, I'd have to watch him walk past his house to the end of the street before he'd vanish and then (after the loading screen) appear at the start of the street again so that he could go home. It looked ridiculous.
So for me, open neighborhoods are the solution. My present sim would be able to hang out with her sister's family without a loading screen, my party sim would be able to walk home at night without problems - but at the same time I don't have to watch sims take forever to get to places that are far away.
Other points I've seen in this thread has to do with being able to place things on the "empty" areas between lots and giving us tools to place lots there - and yes please! No matter the system we have this is something I want. I don't need full editing options, but the ability to put a playground, food stall or bench in an empty area would be wonderful. And I definitely want to have a say in how lots are placed. That, however, is how I see it an entirely different discussion and could work regardless of which type of world we have, open or closed.