The signal received achievement in the first portal is basically the game on hard mode: you have to find the radio in pretty much every test chamber and take it to some other place in the chamber. It requires a much deeper understanding of all of the puzzles and the portal mechanic. It's my favorite achievement and I still haven't completed it yet.
The only other thing I can think of would probably be the final boss in FTL on anything other than hard mode: it's pretty much the most punishing thing I've played in a game that wasn't just complete bullshit behind the scenes coding (like probability modifiers and input scanners).
The only other thing I can think of would probably be the final boss in FTL on anything other than hard mode: it's pretty much the most punishing thing I've played in a game that wasn't just complete bullshit behind the scenes coding (like probability modifiers and input scanners).