From observation, can I expect that a game will get a permanent price cut (or a weekend special deal) X weeks after its launch, or after the sales dropped by Y percent or whatever other reason?
If nothing else, you can be sure that if you pay attention and wait long enough, the game you so look forward to will be on sale eventually. I have yet to buy a game on Steam without a significant discount. I've had to wait quite a bit for some of them, but the vast majority will come on sale after a while.
Because the discounts and prices are negotiated with the publishers, you'd be better off trying to analyse on the publisher level rather than across Steam as a whole.
Additionally, because people would put off buying a game if they knew it was going to be discounted shortly, Steam will most likely purposefully avoid any such pattern (to ensure maximum revenue).
I haven't seen a pattern on new games but, I have seen that Valve will reduce the price of the previous version of their own games (and some that they distribute) just before a sequel comes out.
They also tend to offer discounts/packages on holiday weekends. This is mostly on independent games and retro packages.
Finally, they tend to offer free games when celebrating an event like the release of Portal when the Mac version of Steam was released. In the later case they like to drop hints that something is coming but like to keep the details a surprise.
bad, old, and boring games will get discounts :)
Usually it happened on Thursday, so people could buy them on Friday to play at weekend. Often presales are cheaper, because people don't know how bad is game, so they buy it to try. Another time is when most players stop playing multiplayer game, so making 50-70% discounts bring more people(and money) as servers are already working and there is capacity for players. And one more case: when new game is almost released (e.g. civ5), then previous versions get discount, because almost nobody will buy them after release.