I know that many modern apps follow this pattern and it may have to do with making things responsive and work well across many different devices and platforms, but I simply cannot be happy with low information density.
YMMV. It's good that there are so many different clients to chose from. IRC still rocks.
That being said, The Lounge supports themes, and it's trivial to reduce spacing to fit your needs. There's even a box in settings where you can put custom css directly in.
I mean, HexChat use roughly the same space (the username part is configurable), so this is hardly a problem of 'modern UI'. (XChat, which HexChat fork from, was initially released in 1999)
That being said, I think the whitespace between the lines was the complaint (IIRC the default is a line size of 1.4 and 3px padding)
With a little CSS I eliminated the WS, and here's a screenshot with the two sidebars hidden:
Not the guy you asked, but isn't that the default? I've always done '/names' to see who was there and '/list' to see what channels were available.
I personally find most "slack-ified" IRC clients to be much more legible and easy on the eyes than weechat/irssi although I do like the terminal clients for their keyboard nav and even deeper theme options.
But obviously the best part about The Lounge being browser-based and overall more modern is that I can easily use it from my phone, too. SSHing into an irssi session is far more tedious and annoying when I'm not sitting in front of my desktop.
Using this on a channel with 200 people in the channel and 2-4 conversations taking place at once, and they matter a lot.
I think with thelounge the problem can be solved with some custom CSS ( which thelounge supports adding ). But this is not a very user-friendly option.
Funny enough, just like at a previous online book company I worked at, the problem can also be solved by a greasemonkey script :p
/* remove padding for channel switcher */
#jump-to-container .channel-list-item[data-type=lobby] {
padding: 0px;
}in the custom stylesheet section of your thelounge installation.