I see a few issues.
There is no contrast or break between one post and another, yes you have the grey profile bar, but it's very light, and with no sides, it doesn't look like a break. Every time a chat forum has done that, they had issues with miss-quotes and people being frustrated (been doing this for many years). The other thing is that everything looks like it's just floating. It all lacks definition. Also, with no breaks or borders, once you scroll past the top, it looks extremely bland. Oh look, a white page with text, what is this, an e-reader? To me, it looks like the images didn't load properly, even though they did. It looks more appropriate for mobile device theme than a desktop theme. Which is what it looks like you guys started with.
Now compare that with the header and footer which has lots of blocky, brightly colored images images, and it looks like it all just fades off into nothing.
The thread title area (where it says Give some reasons) also looks a bit big. The same goes for the post a reply and likes indicator. They take up a lot of room for no real reason compared to how everything around them looks tiny. Look at the message box to the right, it's utterly empty because of how stretched out everything else is.
If you want to stick with the huge title, try letting the title run all the way across and put the pm area on the line with the reply and likes after you compact it a bit. I don't really need to know when I was last here, or how many total pm's I have, just that I'm logged in and have a PM. Not sure it would work, but my guess is that it would make it look a bit better organized.
Have the designers try some lower resolutions before sticking to such a huge font. That text is going to be HUGE on lower resolution screens. Not everyone runs at 1080 or more. The G on my 25in is nearly 1/2in tall. Excessive? wait till i see it on my 13in laptop at 1200x800. I get that you are trying to prioritize the text, but that goes too far I think. it takes away from the design.
To me, it looks like you upped the font size in posts to make it stand out, but you only did that because you stripped the borders and framing. Once you increased the post font, you had to up the header size. You went backwards. Put the borders back, and use something to separate the profile from the post, then you can reduce the font sizes back down. It's simply too much information without organization, and you are trying to use font sizes to correct for it all.
Also, unless you want noobs making 3 posts in a row, leave the multi-quote, it's a single, tiny button. If that throws off your entire look, you should probably reconsider the look. It's also a hassle to do multiple quotes so even those who can do it manually, won't. A better option would be to just slightly shrink all of the buttons if that is your priority.