I disagree with you on that there aren't enough differences between the difficulties:

"...rare to see a Horde outside of the occasional Crescendo Event, whereas on Expert, it's likely you'll face a Horde around every corner."

On easy difficulty they receive just 1 damage per hit, 0.5 if attacking a Survivor's back. Meanwhile you receive 20-40 damage for every hit in expert.

The AI overall is much smarter on expert and the tank also becomes a mobile witch (instant incaps on any hit).

More info: http://left4dead.wikia.com/wiki/Difficulty

It's a completely different experience in the game you're playing, just like any other game with harder difficulties which people also speedrun on.

Losing to a horde comes down to skill. Like I mentioned before: you're likely to face hordes on every corner in Expert. That's just the nature of the difficulty.

Sure, the run may overall be slower than an easy one but that's why it should be in a different leaderboard/category.

So why not have both, get the best time for all the campaigns or individual ones and to show off that the time you just got was on expert. We shouldn't be having easy and expert runs on the same leaderboard when they are so different.

There hasn't really been an interest in doing glitchless runs on this site, but I've seen plenty of comments elsewhere where people frown upon exploiting so much on easy difficulty. Most of the pro competitive players that I've seen certainly think so at least.

Yeah, I think I've been convinced at this point to remove the lerp part all together. I guess you're right about it not giving that huge of an advantage. It makes it less jerky for special infected and more jerky for common infected. It's a bigger advantage when playing in versus than against bots if anything IMO.

I feel like so many people already use lerp I feel it's unnecessary to change it specifically.

Yeah, net_graph does show lerp.

Here are some comments from the Left 4 Dead 2 subreddit who didn't bother creating an account:

3yebex (Twitch.tv/3ybx): If it's a huge advantage and anyone can use it then why don't you just damn well use it. There is zero need to have a different category for something like this, which is actually a preference and not a requirement.

There is no need though for a separate category for the special snowflakes. More categories will just add to confusion for people looking through.

The only thing I would want a separate category for is current and past versions. Using the most recent version should be a category separated from using the older versions.

HighCookie: This game is too old for people to still not understand /use lerp. Lerp makes no difference to the run and you arnt a special because you used 100+ you're just raising the chance of random shit ruining run and its certainly not in the same pool as scripts.

Here's another idea: How about Realism too? I simply see no harm in more categories/leaderboards. :^)