The Scout
The scout is a light class. He can move very, very fast, but he only has 50 green armor and 75 health. He also has weak weapons - in TFC his weapon looks like an MP5, but it shoots nails like the Quake1 Nailgun. The Scout also carries a concussion grenade. When this grenade explodes, it will fling anyone within the blast radius violently away and will cause them to be "concussed". When you are concussed, your FOV wanders around like you are dizzy. This renders your crosshair useless, and it makes walking in straight lines very difficult. Because the Scout is so lightly armored and easily killed, he needs to rely on his speed to get by enemy defenses and sentry guns. Scouts are generally used for flag running, as they do not have the firepower to kill many enemies.
What's new?
In TFC the scout no longer has a scanner. Most of the people I know never really used the scanner that much, so this shouldn't affect game play dramatically. Instead, the scout's "special" ability is the "flagstatus" command. This command gives you information on your flag and the enemy's flag. This isn't really a special ability however; any class can use this bind. Unfortunately the flagstatus will only tell you if the flag is in the starting position, if someone is carrying it, or if it is "dropped somewhere". No specific locations are displayed if the flag is dropped. Usually your team should tell you exactly where the flag is anyway, so this isn't a big deal.
The Scout's nail gun now looks like the Half-Life MP5. The model changes only affects the look of the gun though; it is still a nail gun in its functionality. However as you may know, the TF nail gun is a slightly modified version of the Quake1 nail gun because it shoots only half of the nails as a normal Quake1 nail gun. In TFC the nail gun acts like the old Quake1 nail gun instead of the modified TF nail gun.
The overall movement speed seemed a little slower. It may be my dementia, but I think that it's just fault of the Half-Life engine. In fact, everything seems to move a little slower (movement wise) on the Half-Life engine than the Quake1 engine. Similarly the nails from the nail gun seem to travel slower than Quake1.
The concussion grenade was reverted back to the pre TF 2.8 concussion effect. The blast has the same propelling abilities, but the actual change in your FOV is not like TF 2.9 (where your FOV is constant, but your body stumbles around). TFC concussions now make your FOV sway back and forth, just like the pre-TF 2.8 concs. I really hope this is hack proof - the conc hacks were one of the worse hacks to affect the TF community. On the subject of conc grenades, the timing of all handheld grenades in TFC is a bit slower, i.e. they take longer to go off after you prime them. You can still prime and hold them, but you will have to adjust to the longer fuse time to use all the TF1 tricks. In particular, conc jumping will have to be re-written. Of course caltrops also come out slower than in TF1.
Oh, and a scout on red team looks kinda fruity =]