Don't rush the storyline! Of course people will complain that it's too short if they rush through it. There's actually a lot of painstaking details and secrets that the developers put in the singleplayer that you should really slow down and investigate - and I'm not just talking about the achievements only.

As for the purchasable DLCs, yes it's sucky that some stuff is by additional purchase only, a la TF2. It's not what the entire game is about though. Working and playing together with friends is a wonderful satisfaaction. Would you really care about purchasable customizations that you'll never really get to show off that much?

If I were to reconsider this game, maybe $45 to $50 is pushing it in price, based on the replayability of this game - even after considering user created levels. Difficulty is on par with the previous Portal. Nevertheless it's an immersive singeplayer storyline and gameplay that's worth experiencing.