Ok, this thread got me thinking about what really makes a good TF2 weapon. I thought about it for a little while, and decided that a good weapon has upsides and downsides that enable and encourage you to adopt a new playstyle with that weapon. There's a lot more to designing a weapon than just slapping some neat upsides or a new gameplay style on it, and then tacking on some random downsides that seem to "nerf" it enough to make it not-OP. We all joke about how Valve throws around "No random crits" as an arbitrary downside on all sorts of weapons, but slapping something like "20% slower shooting speed" or "-10% damage" without thinking about what kind of playstyle such a downside would encourage, is just as arbitrary and irresponsible. You shouldn't choose your downsides with the goal of making your weapon "not OP", you should choose them to reinforce the playstyle that your upsides enable. This is the ultimate conclusion I came to:
Upsides should enable you to play with a new playstyle that was impossible or non-viable with another weapon. Downsides should encourage you to stick with this playstyle rather than treat the weapon as a slightly changed stock weapon.
Let's look at a few examples of good weapons that follow this philosophy.
The direct hit's main upsides are more damage and faster projectile speed. With the stock rockets, it's hard to score a direct hit, since you have to lead your target with a (comparatively) slow moving projectile, which he can see coming and dodge with moderate reaction time. The faster moving rockets enable you to score a direct hit much more easily, and that is obviously the playstyle this weapon encourages. Now look at the main downside: reduced splash radius. This makes it almost impossible to kill someone with splash damage, and so it encourages you to go for the direct-hit playstyle. Even though the upsides and downsides are quite different, they both work towards the same playstyle: shooting for direct hits.
Now let's look at l'etranger. It gives you more cloak time, and an opportunity to gain back cloak with damage. This obviously enables you to spend more time invisible and sneaking around, and avoiding encounters where you otherwise might have to pull out your revolver. The downside is a damage reduction, which makes perfect sense. By taking away some of your ability to hold your own in a fight, l'etranger encourages you to use your extra invisibility to avoid fights.
Lastly, the good old degreaser. This one is obvious; the super fast weapon switch time enables you to take advantage of more opportunities to use your secondary and melee, since you can pull them out quickly to finish off your opponent. However, the degreaser does less damage and has a weaker afterburn, which encourages you to use your secondary weapons at opportune times since your flamethrower has less damage output than it otherwise would. It also encourages the playstyle of finishing off enemies with your other weapons since you can't count as strongly on your afterburn to finish off enemies.
I agree very strongly with Dr. Element here. If you're designing a weapon, don't just think of an arbitrary combination of cool upsides and nerfy downsides. Your upsides and downsides should work together to both enable and encourage a particular playstyle that is both different from the regular stock playstyle and is, most importantly, fun.