I don't really know how to describe the feeling but I'll give examples, but the new layout to me makes it feel tiring being on for too long.
1. There is the top menu, then a big add between that and the next part, a big ad between the bottom and where you reply, then on the right column you either have 2 ads on top or some type of summary, then ad, then either related posts or other areas. There are no borders, the adds have the same color and text type but larger font, and it all seems chaotically merged into one thing where you need to constantly sort out what you are viewing. What is and isn't an ad, etc. I know you had 3+ ads on the old system but it seemed much cleaner. I also think maybe the ads were around everything vs in the middle and between.
2. I forgot how many topics were shown per page, but it felt like a lot more, maybe 50 vs the 10 now. I preferred less changing pages.
3. I do miss page views. Sometimes I'd reply or post things for the benefit of lurkers, but I haven't noticed thread views on this new system, so no telling if viewed by 1 person or 10,000, so that may discourage some participation if people are unsure if it's a hot topic or for the benefit of lurkers.
In summary I think atleast for me it might just be information overload. The adds inbetween everything, only 10 topics per page with big summaries of each topic and text put it, that there is a lot of work and text to view for 10 posts then change a page that it's at least for me feeling like a lot more work and somewhat tiring.
What I would suggest to try with the new styling would be having the ads on not inbetween but top/bottom/side/etc. Maybe having some lines or visible borders between things. Have the right consistent vs sometimes text on top and bottom vs 2 ads on top and something forum related on bottom. Maybe reduce summary text for each post or just have it listed like it was before and a person clicks on a topic to view any text on it then allowing much more topics to be listed. Bring back page views.