Sylvan Safekeeper, arguably the best one-drop ever? Not by a long shot.
Sorry, I'd still take Rogue Elephant over that any day of the week. There's a reason R&D felt comfortable dumping it in our laps with a one green mana cost.
As for Spelljack...I can see how it would swing a game pretty well, but does anybody remember Desertion? I don't recall it ever seeing a huge amount of play, and when R&D dropped it into 6E, no one seemed to notice or care. Sure, Spelljack's even more effective, but at a 3UUU mana cost, I just cannot fathom it seeing that much play. It'd probably be great in casual play, but it's also way too easy to work around.
Come on, when your opponent's sitting there with six mana untapped, at least three of which are islands, I wonder what they're telegraphing. If you're playing a blue opponent with that much mana free, I'm expecting them to be able to counter every threat I try to drop before Spelljacking one and letting the bigger one I've been holding get through.
Spelljack's overrated right now.
Fledgling Dragon ain't a whole hell of a lot either. Half the time, by the time you get to threshold and he grows up into a nice healthy Shivan, you could've cast a real Shivan a couple turns ago. Until then, he's a crappy 2/2 flier. Whoa, stop the presses. And before you mention using Book Burning with it to hit threshold, remember that the time you've spent sitting there playing with yourself will be spent by your opponent setting up to knock you down. A 5/5 front-pumping flier doesn't always own a game.
The Brushhopper's good, I can see it getting a lot of play. Just remember what no evasion did to Spiritmonger.
Guiltfeeder's too circumstantial.
I do think Seedtime's a bit overpriced and overrated right now. I think it's a decent card, but it's a color hoser in your rare slot. Against blue it's pretty good, but an extra turn against blue isn't really even necessary; the two mana you saved to Seedtime (which your blue opponent must now work around) would ordinarily have been used to outrun your blue-playing opponent. Playing the countermagic game against blue is a worthless effort. Seedtime's all the rage right now, but I just can't see it really making that huge a dent in the environment past that.