We players invest a lot of time during character generation trying to set up the right combination of abilities, feats, weapons, and powers to make sure we have a high chance of success in whatever situation our characters find themselves in. But once the character generation is done, I don’t think too much about getting new bonuses, especially on a situational basis. The one major exception is combat advantage, which I pay attention to all the time because our Rogue player is obsessed with sneak attacking. And by “obsessed” I mean that he seeks out that extra damage (as he is supposed to), not because he’s a douche who likes stabbing people when they least expect it. 3e used to be filled with tons of situational modifiers: size, height, wind speed, etc. In 4e there’s basically combat advantage and cover/concealment. This is a good thing, it makes it easier to apply the rules.
What this means is that it is left up to the DM to give situational modifiers as he or she sees fit. For example, the Gabe from Penny Arcade handed out bonus cards depending on the crowd’s reaction during a tournament. This mechanic was also used in the PAX Dark Sun adventure. I think these types of situational are pretty awesome: it’s an easy mechanic that doesn’t unbalance things but encourages roleplay and strategic thinking. The only problem is that these are not part of the rules and thus the DM has to come up with them and then tell you about it so you can make use of it.
But who wants to wait for the GM to come up with a mechanic for you to exploit? I think we should be proactive about requesting bonuses (do so before taking a particular action instead of being told “no” after performing that action). Things that might warrant a bonus: having a background or character experience with a particular ritual, having something special in common with an NPC (race, language, religion, etc) especially when that trait is rare (both being from Sharn isn’t that impressive in Sharn), or doing something cool on the battlefield or being in an unusual situation.
There are two levels of bonuses generally available in the game. There’s the small +/- 2 that is a useful but not-unbalancing boon, which is given for difficult or exceptional situations. For example you can expect a +2 for cover, combat advantage, or from a racial bonus or background. A +2 is probably going to be what we’d normally expect to get from doing a cool stunt, knowing the NPC’s mother, or belonging to a certain group. There’s also a +/- 5 that one gets for truly difficult, rare, or super-human events. A character can will get a + 5 for total concealment or training. +5’s are generally not something that can happen with a little situational context. For a +5 to a diplomacy, I’d expect to have saved the NPC’s life. I’m not sure what would merit a +5 in combat without applying a status that already grants that +5. Maybe some sort of magical effect or unique terrain?
I think for any group there will have a be a learning period to discuss what sorts of things your GM thinks is worthy of a +2 or a +5, but hopefully you have room for negotiation. These should be situational and you shouldn’t build a character around using them or abuse any bonus, because that would be unfair. Besides, it’s easy enough to get combat advantage and the like already. Do you play with an established house rule for situational bonuses?