Hehe. Remember that I have a strong bias against wimpy (1/1 or weak 2/2) creatures, and tribal involves a lot of them. Outside of the vastly overpowered 'tap 5 X card' monsters, most of the things tribal cards can do for you are easily obtainable elsewhere as well. I do like creature theme decks---I have a very strong (see mostly untested) angel/dragon + cleric/barbarian deck right now, but it doesn't rely on some of the weird stuff they have tribal doing in Onslaught. (Side note: I was really disappointed to see that Onslaught didn't have barbarians. I was looking forward to getting something better than Halberdiers to be the cheap basic barbarian card for my deck.) Overall tribal relies too heavily I think on the player being able to get out a large number of creatures of the same type, and I'm skeptical of how easy that will be to accomplish. The big thing it does do is provide a way for small weenie creatures to compete with the giants of Dominaria if they're allowed to grow in numbers.
As far as justifying my skepticism of Morphing goes, it has a lot to do with how I envision Morph deck games proceeding. The scariest Morphers are those which do something special if not blocked. Forcing the damaged opponent to discard two cards for instance. The problem I see is that unless your Morphers outnumber his creature force, (I'm assuming this force is composed of fairly normal creatures most of us would have used previous to Onslaught, or creatures similar to them. My favored Daru Lancer is a great Morph blocker.) an assault still isn't worthwhile. The average Morph card is either weak or expensive to Morph. Unless the game has gone on a long time (giving your opponent time to build defenses) you won't be able to fork out the Morph cost for more than one strong or two weak creatures. Thus, you may kill one of your opponent's blockers or get a sneaky blow in, but if your opponent has a defense like the ones I make, your Morphed creature is probably very close to the only one you have left. Even if your opponent has to discard, he's still in the superior position. I see a different scenario if you have tons of extra mana or if you outnumber your opponent with your Morphers, but I choose not to rely on such fickle factors. A well designed Morph deck with some selected non-Morphing cards to draw your opponent's attention could avoid this problem I suspect, but if played against an average defense from an average position with an average mana supply, I see a Morph deck losing due to mana scarcity. The Cyclo-Deck I discussed above could easily handle a Morph attack if it was at all set up. If the Cyclo Enchantments I had out were mostly Astral Slide or Lightning Rift, I'd cycle a card (keeping one always in my hand for that purpose) I'd Slide or Rift a few of his attackers before I blocked, spoiling the surprise factor and taking the heart out of the enemy assault. Alternatively, if Invigorating Boon was my dominating Cyclo-Enchantment, I could simply add +1/+1 counters to the creatures I expected to come under heavy assault, hoping it would be enough to put the beat-down on whatever it was those creatures blocked. While it wouldn't be a perfect solution to the Morphing enigma (don't get me wrong, I think Morphing has some strong points to it...I'm only expressing some of my reservations) I expect the (nonexistent) Cyclo-Deck to work well against a Morph deck.
Like Fell said, though...it's all just theory at this point.