How did you not know this, lol? Q start is mandatory anyway. That stun provides 10x more utility than any other ability. You can't really make any level 1 plays with W.

start w on leona? -1

I have to disagree about taric. If you go in on him, it's at least 1.2S stun on you. Also, I agree with the first 2 comments, the health given from the ruby sightstone is really awesome! Plus, I think cait goes with leona pretty well, early game if you land an e-q combo, she can use her q to do a ton of damage.

And after level 6, i think leona is great with any adc.

^ When they place the ward its visible for a little while. You Auto>Q>auto and it dies. (Q resets the attack animation which lets you get out two attacks super quickly and then you can slip in a 3rd one while you'd normally only be able to get 2 off)

Explain? Breaki wards without pinking? Waaaaat?

I firmly believe that Taric is the worst zoning support in the game out of all the zoning supports.

I'm sorry to say, but ...

... This really did hurt me.

Here you have some maths on Sightstone: http://www.mobafire.com/league-of-legends/blog/vynertje/get-off-my-ad-carry-support-101-part-2
The information given is somewhat outdated but the maths are still correct.

Shield of Daybreak lets you break wards without pinking them, which is WAY more valuable than Zenith Blade at level 1. (You cannot follow up afterward if you take Zenith Blade and you aren't tanky without Eclipse).

Plus, flash triple auto takes half health from squishier champions. If your AD's level 1 burst is formidable you could get a surprise first blood before they can flash away, if you managed to poke some health down earlier. (Easier to do when their support took Ignite instead of Exhaust).

Regarding the masteries, I'd suggest putting some points in the utility tree to get Greed . Two extra quints isn't bad at all in my opinion.

I think people take either Zenith Blade or Shield of Daybreak because they both provide CC(root or stun). Eclipse sure is a great ability, but for level 1 trades or chases I think it's better to get one of the other two. Shield of Daybreak is nice if your opponent is stupid enough to face-check the brush you're in, if your carry is bright enough half of the hp of one of your opponents is gone before the laning even started. Otherwise I'd recommend Zenith Blade because Leona isn't good at getting in range without it, so Shield of Daybreak is rather useless then. I don't take Zenith Blade to go full ****** at level 1 but for the root and a passive stack without having to wait for 3 seconds. Also, if you take either Shield of Daybreak or Zenith Blade you can start going all-in at level 2 if needed.

Regarding Sightstone, I think it's about preference. It sure isn't needed on any support, but I myself tend to find it more useful on Leona than on Nami, because Leo really loves the hp. Sure, there are better hp items but this item is just very cost-efficient. Buying wards next to it is required, of course. I believe we've had this discussion before, so I hope I didn't light the fire just now ^^

Minor nitpicks:

Taric can't zone?

You should consider using ad runes on leona, personally i find it really useful especially after you auto, then q to reset your auto attack. Another thing I'd suggest is potentially putting ignite on leona, it's one of my favorite summoners to use on several support champions