Summoner has always been my favorite class in every Final Fantasy I've played, so I can't wait until I see FFXIV's implementation of it.

I think in FFXI (correct me if I'm wrong, since I only played it briefly), they constantly ticked down your MP while a summon was active. I seem to remember reading that doing that made it so that actually keeping out a summon was undesirable, since it wasn't as easy to gain back MP. I think that system might work better with FFXIV, due to their constant MP regen, but I kind of wish they'd go the route of Dragon Age and just make it so that a portion of your MP is inactive while the summon is out. Let's say, 25% of your MP is 'held' while you have a summon active.

As for summon skills, I think the summoned creature should have a list of it's own skills that it could use, utilizing it's own MP, but also have skills that require an expenditure of the summoner's MP in order to work. I think cool little combination magic effects would be awesome.

Not sure how directly controlling the summon would work, but if you wanted it to be a once every so often big cast like it is in a lot of Final Fantasies, you could make it so that when you summon a creature, you take control of it, but lose control of yourself, like gaining a permanent sleep debuff while the summon is active. Would also make it so that aggro would constantly build on your sleeping form, so you had to work to keep up aggro with the summon or risk your sleeping form getting demolished. Think it would be kind of interesting.

Anyway, I doubt that summoning would be a skill accessible to other classes, but, I'm not sure what the future will bring, so I could be wrong.