I was a bit late but muratakburak did. If they mentioned it we would have known that, I think :P

Not all slots are pre-determined. For example, there's only one guaranteed spot in SWERC. The second place team usually goes as well, but this is only determined afterwards.

I believe that results in different regionals are analysed and they give additional invitations accordingly (meaning that if our 2nd team is much weaker that 1st, they might invite another team in a different region).

In a way yes, but actually we didn't even work for "it". After high school none of us practised. It was the first contest we participate together, actually it was the first "problem solving" we did together :)

You may be confusing bits of two or three separate occasions here :)

2011/12: Because the next ICPC was to be in Warsaw, the Polish Collegiate Contest (AMPPZ) was granted one slot, which would be awarded to the best university from AMPPZ which does not qualify from CERC. We got a good (4th) position at AMPPZ, losing only to University of Warsaw teams, so only two things could make us not go to World Finals: University of Warsaw not qualifying from CERC (highly unlikely, especially seeing that top3 on AMPPZ was all Warsaw) or a different team from our university beating us on CERC. At CERC, we performed really poorly and did get beaten by another team from our university, but their position was also not enough to qualify — there were 3 slots and they were the 4th-best university. So we were happy about going to the WF. However, then an an extra (4th) slot for CERC was announced. There was some interpretation-of-the-rules thing at that point, but in the end the other team from Wrocław got awarded the WF spot, and our spot from AMPPZ was awarded to the next university (Technical University of Poznań). Not nice, but reasonable.

2012/13: We got one place behind the four WF slots (even though we felt we performed very well, many CERC teams seem to have done an excellent job that day). We heard unofficially that they were considering adding one more slot. Then, after about one month, we did indeed get the invitation :)

2013/14: Again we got the fifth university-place on CERC. But at the closing ceremony it was announced that there will be five slots. Great :) Then, over the next month, we did not get the invitation, became gradually more worried, then learned unofficially that CERC would get four slots in the end (even though in the previous year it was 4+1, and the region was doing crazy well on WF). Two months after the regional, we appealed in an effective way and pretty soon the ICPC made it right (but in the meantime other people involved also tried to appeal on our behalf and were unsuccessful).

So my impression is that there is not much ill will, but the process should be made fully transparent. The ICPC does use some criteria that they talk about (medals from the region at the previous World Finals, for instance, and some perceived level of the regionals) and a lot of criteria about which they do not. And the Asian regionals frankly sound like a horror story to me, and possibly highly corrupt. I would say that every year, before the first regional begins, some formal description of the rules should be made available to everyone (maybe even an algorithm) which, given the results of all regionals, will determine the World Finals advancers without any further human input.

This does not sound like a pipe dream to me, because the ICPC is in fact becoming more and more open and modern (in 2011 or 2012 they have started to release tests, for example, and did you know that they are now releasing this stuff as well, or this?). Maybe in 2-3 years...