Series 4 isn't a least liked series of mine atm, but series 3 was definitely better. No Catherine Tate, and Freema kicked frikkin' ass. The finale's looking good, though... this wasn't the worst way to bring back the Daleks (series 2 finale already did that one) and it does help explain why the Doctor can't just go back to the start of the Time War and save everyone.
I don't think there were continuity issues with Eccleston leaving - he'd always planned to leave after a single series. :/
Anyway, I'm tempted to believe that the regeneration will be undone with a Deus Ex Machina at the end of the episode - the whole Earth being invaded in the present time frame by Daleks is a bit too extreme, and the series - including Torchwood and Sarah Jane Adventures - have tended, to a lesser extent, to fix these kind of issues with some kind of time paradox (Okay, I dunno about SJA, but it happened at least twice in Torchwood and once in DW). I'm thinking some special effect of the Medusa Cascade maybe?
(If you think about it, it's kinda supported with how likely Gwen, Ianto and Sarah Jane are likely to die in the next episode... there will be more series of both Torchwood and SJA, so they can't really have their flagship characters dying in a different series first)
If I recall, Tennant's already been signed to do the Christmas Special and the next three specials after that... but I'm not sure if he's on to do Moffat's first series. Either way, he's staying, don't worry.
In other points: Christ, almost the whole gang's back now: all that's missing is the guy with the opening-up skull from series 1, Mickey, and Martha to get her ass back to the Doctor.
I think the Daleks have been downgraded somehow - they got hit by both bricks and a paintgun ball. Normally their shields protect them from just about everything, even if it shouldn't be a threat. So maybe Gwen and Ianto do stand a chance against a lone Dalek?
And Jack's got his teleporter back, but I'm guessing he's lacking time travel still. That's frikkin' awesome, but I'm guessing it's gonna be undone by the end of the next episode, so he won't have his 'port in Torchwood.
Also, does anyone else think that Rose's teeth look damn ugly right now?