No sense burying the lead here. I think Populate is a cool, well-designed mechanic, but likely to be awkward-to-terrible in EDH. Token decks tend to focus on amassing hoards of 1/1s or other small tokens, overwhelming defenses with quantity rather than quality. Thus, Populate will likely have negligable impact in those sorts of decks, where adding one more 1/1 to your teeming mass of saprolings will hardly matter at all.
Using Populate to make copies of bigger, better tokens has some potential. In particular, copying token-copies of "real" creatures. For example, I like how Populate interacts with Mimic Vat or Cackling Counterpart - where you're getting not a generic 0/1 Goat or something, but rather an extra Shriekmaw, or Eternal Witness, or a Duplicant, or a Bogardan Hellkite, etc. The problem here is, I don't think there are enough good ways to make tokens that are valuable enough to be worth Populating.
I hope someone proves me wrong, because I am a Selesnya fan at heart, but I just don't see it.
Rant aside, let's get on to the cards.
I kid, I kid. This is a pretty cool design, but I don't see Trostani making an very good general. I'm definitely not as down on lifegain in EDH as many people are - I think it can be very relevant.
If I were going to build around Trostani, I'd probably focus more on the lifegain than the Populate, for better or worse.
Here's one Populate card I expect to see get a fair amount of play. Mostly, though, that's because this isn't just some existing card with Populate tacked on - the only other card in the game that offers blanket Indestucability is Dauntless Escort. This card is even better, as your opponents won't see it coming.
At three mana this is a steal. I'll play this in decks that have zero chance of using the Populate portion. Hell, I'd play this in some decks if it cost FOUR mana, and just lost the Populate. Spending three mana to ensure your guys are the only survivors of a Wrath is awesome.
HA HA HA HA!!!
I usually find "Fog" effects to be terribly uneffective in EDH, and this looks to be on par with most of them - bad. That said, my Edric deck makes use of some of the better "fog with upside" cards, like Constant Mists... I just don't think Populate is enough of an upside to compensate for the inherent weakness of the card itself.
Goodness, that's a LOT of power and toughness and Trample for six freaking mana. The fact that I can't play this in Stonebrow causes me such angst and sadness, you will never know it's like.
As is, I really wanna do something with this, plus Restoration Angel, Vesner the Sojurnor, Cackling Counterpart, and Rite of Replication. Start stocking up on those Wurm tokens now.
In case it's not clear, this is very, VERY playable. Doesn't even have to be a token deck. Pretty much any deck in the right colors could feasibly play this.
Is Watchwolf a good EDH card? Nope.
Every Rhys deck ever, no-brainer. Great for Ghave, too, if you want to go "more aggro, less Grave Pact" with your build (certainly a valid move!). My current crop of token decks (Rith and Ghave) would both rather have either Mirari's Wake or Cathars' Crusade than this 90% of the time, but if you really want to push for speed and lethality, I'd certianly suggest trying this on for size.
Yeah, remember how I said I was more open to lifegain in EDH than most? Well, I still think cards that do NOTHING but gain life are shit in EDH, unless they're Becaon of Immortality, or in a few decks, Congregate.
Meh. If discard is a very real component of your metagame, you could get mileage out of this, but for most groups, this is essentially a vanilla 4/4 for 3, which is just a bit too vanilla to make the cut in most groups. For low-powered, aggro-friendly metas, then he might be okay.