I don't mean to sound mean (unintentional pun), nor do I hope to sound like a fanboy, because I'm not, but be happy Left 4 Dead is getting a good sequel in a short amount of time.
Left 4 Dead is an awesome game, don't get me wrong. It was really fun to kill incoming zombies running at you. (If I could only count the times I suffered at the hands of the Tank..) But I'm sick of hearing all of these complaints about the new game.
Using the boycott's manifesto as a guideline, I'll tell people why they shouldn't complain.
1) Significant content for L4D1 was promised, and never delivered.
What's content from one game compared to a whole new game from the same series made by the same developers?
2) Valve put little faith in L4D1 since they almost certainly started working on L4D2 right after release.
Most people complain about the long time it takes to make a game (ex: Duke Nukem Forever, which got cancelled) You're getting a sequel in 7 MONTHS!
3) The fact that L4D2 is nearly identical to L4D1 will decimate the community for both games.
Nearly, but not exactly, identical. Be glad Valve is not doing what Sega is. (ex: Ruining the series, like Sonic, or re-releasing the same games over and over, a-la House of the Dead series)
4) The announced date is not nearly enough time to polish content or make significant gameplay changes.
It's using the same engine as L4D. All they need is a great story, good levels, new characters, weapons, etc.
5) The new character designs seem bland and unappealing so far.
Key words: so far. All games go through development. This may be the forerunners to the actual designs. However, I do agree: the characters seem bland so far.
6) L4D2 is too colorful to fit in with L4D1's visual aesthetic.
Think of setting. L4D took place in Pennsylvania in what i assume is winter. Of course it's going to look dark. L4D2 takes place in the South, in Summer. Of course it's going to look bright.
7) The fiddle-based horde music is extremely disliked, though the differently orchestrated music is otherwise welcome.
Again, think setting. It takes place in the South. Stereotypically thought of somewhere with square-dancing, which I think has the fiddle. (I did like the original horde music, though)
8) L4D2's release will result in a drop in quality and frequency for L4D1 content, even compared to before.
Of course that's going to happen. It happens to most games, like Halo 2. When it came out, Bungie released downloadable content for it. When Halo 3 came out, Bungie released DLC for THAT, and "abandoned" Halo 2. It happens.
9) The community has lost faith in Valve's former reputation for commitment to their games post-release.
You mean to tell me that you're boycotting L4D2 for that? L4D2 ruined Valve's reputation? Not likely. Are you forgetting the other games Valve released? Left 4 Dead, the Half-Life series, Portal. Those are good games, and releasing Left 4 Dead 2 early destroyed what Valve has done? Again, not likely.
Left 4 Dead 2 will be a great game. As said on a related forum, "I hate this hate for Valve" (which is really well-written), "fanboys need something to whine about".