Holy crap, Serious Sam HD is gorgeous. It's jaw-dropping. Everything is sharp and crystal-clear, in a way that only this sort of endeavor can be. If someone had tried to tell me how awesome it was before I saw it for myself, I'm not sure I would have believed them, so I'm afraid any word from me on it would be inadequate. But, uh, yeah. Visual splendor. Indeed.
It's pretty much my first experience with the game too. Almost. I tried the original some time after watching the LP that... errr... who made that LP again? I can't find any trace of it. Anyway. I tried it back then, and there were a couple things that bugged me, like how most enemies just warp in. But I did like it. I actually torrented it; I couldn't find it sold for under 20$ and wasn't sure if I wanted to spend the money. I actually thought, "I got some games I like and haven't finished, and they're prettier, and I already paid for them", and then deleted the game. I'm glad I didn't buy it back then after all, since the way things are now I got a better version and I saved two bucks.
One unique thing that I like about it, other than the crowd factor, is actually the OBVIOUS TRAP IS OBVIOUS element. It's something you don't ever see in a shooter. Sure, sometimes a game will tie an enemy spawn trigger to an item pickup, but that's usually an exception, an occasional dick move. In SS, it's the norm, and it introduces a peculiar risk vs need dynamic. Any given pickup can instantly cause over fifty alien rape machines to teleport directly into your rectum, and after a while it stops being a terrifying annoyance and just becomes a fact of life. If I decide to grab a +100 HP heart, there's the understanding that a gate to Hades will open and that I'll have to earn the right to come out of the fight with more health than I started it with. And I'm cool with that. I'm perfectly cool with that.