That's the magic of me.
At the moment, I'm not looking forward to it because my new computer sucks and runs old L4D like shit, so I won't be digging into this until my old one gets fixed.
When that magic time comes, I am looking forward to the new Valve-designed maps, the new types of infected (special and normal) which will change how the game is actually played based on what you have to do to handle them and how you have to play as them, sorta-looking forward to the new melee weapons (though not much, really), and any lobby/hub improvements that usually come with a sequel but aren't really worth mentioning. I hope they come, because even with the changes to L4D's lobby, it still feels kind of clunky. I know that Valve probably thinks it's streamlined compared to Team Fortress 2, but at least with choosing a server there, you have more freedom.
Is this going to be Half-Life to Half-Life 2? Of course not. But it's going to be too much to simply patch, nor should it be just a patch. That said, feeling L4D is fine as-is and not wanting to buy a sequel because you're happy with it is not the same as boycotting the sequel because you didn't feel the original lived up to expectations. I'm not begrudging your choice, but it is a step-up from the last one. If they changed a lot to make it what you seem to envision as a sequel, wouldn't that be breaking said perfection?