There were some people wondering how much XP is necessary to gain a level on the site. 
 
Here are the values for the first 30 levels:

 

I am now coming up on my 7th S-Rank, so I decided to recap all of them and give a bit of feedback on their difficulty and maybe a note or two.

So here goes - all of these are Steam S-Ranks:

The Misadventures of P.B. Winterbottom - This was my first S-Rank and a ton of fun. You can get an S-Rank on this game in a couple of days, you will need some wits and a bit of dexterity. It can obviously be made way easier if you watch walkthroughs for the challenge levels.

Portal - This one is straight up impossible without walkthroughs. No one is good enough to figure every last glitch in this game on their own, and you will need them. This game is ridiculously hard even with the walkthroughs. I am seriously proud about this one. The game also glitched on me on the last achievement and I had to restore the game directory from backup to get it. There was a Steam patch that should resolve the issue I had.

Modern Warfare 2 - You will need a buddy for this one, a seriously skilled buddy too. This can be a problem on PC, skilled players prefer MP over SpecOps and I had to scour the official steam group chat for anyone willing to help me. The Single Player achievements are quite doable, the last few Spec Ops missions on Veteran are pure pain. This was, overall, a very satisfying S-Rank for me.

Half-Life 2 - This one is bugged, even though I have every achievement in HL2, it still doesn't count as an S-Rank. Let's hope the top men fix it soon. All of the achievements are very doable, especially considering that there is no difficulty requirement. The only problem is that the whole thing takes a lot of time.

Half-Life 2: Episode One - This is the easiest of the Orange Box S-Ranks, with only one really hard achievement - The One Free Bullet. This one requires a full playthrough, and it will force you to really learn to use the gravity gun.

Half-Life 2: Episode Two - This is a pretty hard S-Rank. It includes one of the best AND one of the worst achievements ever. The best - Little Rocket Man, which is quite hard, but very funny and very satisfying. The worst - Get Some Grub, which will force you to check a walkthrough every 10 seconds. The rest of the achievements is fairly easy if you concentrate on them.

Plants Vs. Zombies - This is my upcoming S-Rank - I should be able to finish it today. It will consume quite a bit of your time, and if you are already sick of Tower Defense games, it can be pretty painful. Don't expect much of a challenge, but some of the achievements require a lot of luck. Save game manipulation should get you past these with much less effort.