It also has 1s and 2s merging with each other but not themselves.
The bottom line is that Threes is a much better game than any of the clones I've played. There's a subtlety and polish to the gameplay that is missing from the others. It's obvious they spent a lot of time perfecting everything from the tile distribution to where they spawn and how they merge.
It's very polished ... but. The start animation? Needs to die. Showing me a game which then turns out to be finished when I try and swipe? Bad UX. Score totalling animation? Cute the first couple of times, very annoying thereafter. Swiping and signing? Again, cute for a couple of times rapidly turning into a rancid waste of time when you want to play again (yes, I know you can set this to autosign now but ...)
It's just not smooth enough from start -> play -> restart -> restart to be a good casual gaming experience. It's start -> thud -> thud -> play -> thud -> thud -> restart -> right, back to 2048 then.
When they take up a substantial fraction of the time playing a casual game? Yes, they become important. If I can't pick it up and quickly play N games whilst I'm on the train of a morning, I'm not interested, sorry. If I wanted long load times and faff between games, I have a whole Steam library full of that.
So you have to tap twice in between games instead of once. If this is anything approaching a significant fraction of your time playing Threes, then you are terrible at it. It takes a good 10-20 minutes to get up to a 768 or 1536.
Oh and "< back" from Options leading to the game scrolling down from the top? Very annoying. It has a left arrow. The iOS convention is that going back up the stack means moving left. But no, they've gone for a left arrow with scrolling up and down. Buffoonery.
The bottom line is that Threes is a much better game than any of the clones I've played. There's a subtlety and polish to the gameplay that is missing from the others. It's obvious they spent a lot of time perfecting everything from the tile distribution to where they spawn and how they merge.