This looks really great, I would have built an insert for the planter boxes so that you could have a completely flat table for playing cards or whatever. Obviously you could still add this on.

I'd probably just use a cedar because of the water/rot resistance.

Those gaps do help letting the water and crap through because lets face it, an outdoor table is going to get rained on, there will be leaves, bugs, bird will poop on it, etc. Keeping that sucker clean is the number one way to prevent rot. You don't particularly have to put the gaps in the table, but the gaps let rain and leaves etc pass naturally through instead of accumulating on top.

edit: spelling.