Q2: What makes the Ghost progression so super-fantabulous that it warrants a place in someone's sig?
A 2
The fact that you don't need to take the levels in succession, or even all of them. So you could be incorporeal for 1 LA, and then buy that LA off. If you feel like it, you can then take a second level and buy that off, and so on. The perks that being a Ghost gives you are already pretty amazing, but this makes the harsh LA child's play.
LA + 1)You're normally incorporeal, but can still target yourself with corporeal-only abilities via the ethereal plane. Because of your ability to go ethereal at will, you can avoid many of things that hinder incorporeal creatures (like a limit of going 10' into surfaces). You have perfect fly speed. If your cha is high, you've now got a really good AC. Plus, undead immunities, and if you're using taint, one or two bonus feats.
LA+2) You get CL 12 telekinesis every d4+1 rounds. Or some other power if you're lame, I guess. This can be gained at ECL 3, and is enough to decimate anything approaching a level-appropriate encounter. (firing only normal bolts will get you a ridiculously good damage potential). Telekinesis also is an excellent utility spell. Also, CHA+2.
LA+3) Once you can succeed on a DC 16 level check, you're immune to death. There are several abilities you can use to get rather substantial bonuses to this check (surge of malevolence, for instance), but just getting 15 hd works too.
LA+4) More charisma, and your choice of greater ghost power (or two lesser powers). They are all pretty cool. There's a few extra powers from faerun or libris mortis you might be able to add as well - major image at will is nifty, even if it isn't quite as powerful as malevolence.
LA+5) No. Just No. Never take this level ever.