WARNING: I'm not sure if the following notes are correct. Comments welcome!
Spell interrupts are a fun thing. Interrupts are game-winners in PvP and are essential in a variety of PvE encounters. Druids aren't usually considered master interrupters; that's shamans and rogues. But in fact feral druids have four different abilities that can interrupt spells. The trick is they're all different. As near as I can tell we only have one true interrupt: Feral Charge. But we also have a lot of other quasi-interrupts.
Incapacitates: Maim. Looks like a stun but is a bit different: any damage will break the incapacitate effect. As a side effect, a successful incapacitate interrupts spell casting. Maim is the quasi-interrupt I use most, but if a mob is immune to incapacitates Maim will not interrupt casting.
Spell interrupts: Feral Charge. A true spell interrupt, it stops casting as a direct effect and locks out further casting for a short period. The tooltip "interrupting any spell being cast for 4 sec" makes it clear that Feral Charge is like Kick, Counterspell, or Earth Shock in being specifically a spell interrupt.
In PvP pretty much all four of these interrupts can be used interchangeably. But in PvE most important mobs are stun and incapacitate immune, so the only PvE interrupt we have is Feral Charge. Which is very difficult to use since you have to be out of melee range to initiate it. BTW don't be confused by your combat text saying "Immune". Feral Charge also has a root effect which many mobs are immune to, but the interrupt may still be working.
The game-changer for druid spell interrupts is the Season 3 PvP gloves (also S4). They have the tooltip "Causes your Maim ability to interrupt spellcasting and prevent any spell in that school from being cast for 3 sec." I don't own a pair, but that seems to add a true spell interrupt to the incapacitate effect of Maim. Which is an enormous buff for PvE druids. As I said before; if 3.0 weren't coming out soon I'd be PvP farming up those gloves. But I believe the 3.0 talent trees include more feral spell interrupts, we'll see.