Forums are the biggest thing I'll miss and by comparison the nearest thing for kids these days is probably YouTube or Reddit. However they are nothing like Forums in ways.
How I'll miss spending months earning my way to become a mod, talking about the community or a user in private sub forums, joking/gossip threads with other mods, talking about real life issues with Mom-Mods and Dad-Mods in private forums, and eventually meeting them irl. Customizing my profile with weird ass sigs, animated avatars and coming home to an organized list unread threads interesting to me. Sounds strikingly like Reddit but nothing like it, how strange.
MSN was awesome, but I think kids these days have better options here. Snapchat come close in some ways and does well for the mobile platform.
I felt the Internet was a smaller simpler place with lesser influence from cooperate agenda, state sponsored ideas, and other influencing ideologies and actors. The Internet has become less transparent, commercialized (not just ads) blurring lines between genuine and insincerity.
Social marketing companies specialize in astroturfing places like Reddit making it almost impossible to spot sponsored content sometimes. Try doing that on a Forum and you'll be spotted miles away and made an example of.
Examples of propaganda factories in Russia or Israel with high influence on social media is an example of one big problem Reddit or YouTube has today. It's far too easy to game larger social media networks and make it seem like the consensus.