The biggest thing the druid brings is Earth & Moon.
Once your Wrath or Starfire successfully hits the target, it is debuffed and takes 13% extra spell damage.
The increased crit chance, is the same thing as you get from a Shammy on a crit, and in the next patch it will be a constantly on buff, so that is redundant for the team.
Still 13% spell power is huge, and on that alone the druid is a very strong addition to the team.
Similarly.... Totem of Wrath, Wrath of Air, Tremor Totem and Healing Stream/Cleansing/Mana Spring totems are huge, not to mention Bloodlust/Heroism.



For trash, the druid will probably just spam Wrath.
There is no point in using dots, for anything that will be dead in 2 seconds.
Also with such a fast casting spell, you'll almost immediately get the Earth & Moon debuff on the mob.

For bosses, the druid would ideally have two castsequences.
They are almost identical, except for the fall through.

a) Faerie Fire once, if you've talented for 3% hit (probably won't need this at all in heroics).
b) Insect Swarm, recasting as it expires (highest priority spell).
c) Moonfire, recasting as it expires.
d) Spamming either Wrath or Starfire, until you need to refresh a dot.

The easiest way to do this is with two castsequences or click macros.
One for Wrath as the fall through (spell that casts, when other spells are on cooldown/ticking away).
And the other with Starfire as the fall through.
If you go this route, you'll need something like MSBT to announce procs.
NoLife made a video showing how to configure it: http://www.youtube.com/watch?v=Fvt-cmKWWbY&hd=1
When you get a proc, start casting from the other sequence.

Then you put the dps castsequence or click, or the pallies 96969 sequence on both keybinds.
So you can be using either the Wrath or the Starfire macro, and everyone else is still casting their preferred macro/spells.



You can approximate the Eclipse buff.
It won't be as strong as managing it, but it will be a lot stronger then ignoring it.

I'd build a single click castsequence.
And then go with a castsequence as the fall through.
Something like Wrath x4, Starfire x3.
You don't even need a reset on the fall through.
The point is to cast a few of one spell, to trigger Eclipse, and then several of the other spell.
The trick is few enough in a string, that if the first cast procs Eclipse, you don't waste too many casts of the wrong (same) spell.
And enough casts in the string, that you get the Eclipse bonus on several castings of the other spell.



A 13% increase in spell damage is fairly huge.
I'd try to make the druid work.
The team has worked for you, for 79 levels, so you're familiar with your hotkeys, spells and abilities.

If you decide the druid cannot work...
I'd recommend a Warlock (any spec) as replacement.
Because, Curse of the Elements is available to any spec, and is also a 13% spell power increase.