The demo will feature approximately six single player levels and 2 multiplayer levels. There will be four weapons in the single player levels and maybe 1 or 2 more in the multiplayer levels. Multiplayer will be strictly deathmatch (or free for all) and will feature some of the usable items that appear in the multiplayer for the full version. The main cast and crew plus a few extra characters will be available multiplayer characters for DM. The demo will be approximately 120MB in size and will feature Bot support.
The truth is, however, that Team Fortress 2 won't be released this summer, although this has little to do with a lack of progress on game design, level design, feature implementation or artwork. What is holding further development of TF2 is that Valve are currently building a new engine to drive TF2, and have been doing so for a number of months. As far as Robin Walker is concerned, this new engine will allow them to do things with TF2 they otherwise couldn't have.
It will also make it more scalable across a wider range of system specifications, both capable of delivering superior graphics on high end systems, whilst also making it possible to play on low end computers. It's not another re-tooling of the Quake engine - it's a completely new beast. When I asked what it could do, the answer was something along the lines of 'A more appropriate question might be to ask what can't it do? The idea is to make sure it can do everything we need it to.' Existing content will be further enhanced by the new engine, and not only will it make new features possible, it will make old features work better than they ever have before.