Thank you all.
Since most of the comments to this thread are personal stories, I guess it is only fair that I give you a bit of mine. With the few slaves I have had, things mostly stayed at level one but, in one case, got deeper as time went by, down to level 3 and with a tendency to leap to 4. This only happened once though, and didn't end up so well (see this other topic for a hint on why: http://www.goreanforums.net/viewtopic.php?f=99&t=4428
I must admit that this "sliding down the pit" is very rewarding, when with the right person, with the sub who really wants it too (most, and quite rightly so, show resistance and shy away from going there), and with a sufficient amount of trust. But it's also a lot of energy and dedication on both sides, and it can be very frustrating when it ends up being just "one of those things" (but maybe I should stop deluding myself and accept, from the start, that, indeed, it is very unlikely it could be otherwise).
Level one is, after all, the most comfortable stage and I do understand that most people stay there: very little is at stake at this level. In fact, from your votes and what you all said, it seems that level one is the preferred option, unless the right person comes along... and in this case it starts to go right down to 3 or 4. Level 2 is only a threshold in our way deeper down.