Welcome to the gang! You'll find masterminds get pretty uber 
My limited experince of masterminds (got a robo/FF to 40) says that
Zombies tend to need summoning a lot, since they have to get into melee. The top level guy (Lich or something) gets some really snazzy utility powers. Oh, and you get an extra pet in the form of the ghost chap, who looks cool and does holds (I think).
Robos, especially high level robos mean gigadeath applied liberally to the enemy - lots of nice area effect powers can make large spawns drop in seconds.
I havn't payed enough attention to ninja and mercs to say much about them.
As for secondaries,
Force Field makes up for your total squishyness with the excellent Personal Force Field as an emergency button. Also, the Dispersion bubble gives you and your henches some nice mez protection. Also, as a nice utility power, detention field is great to remove a significant threat for a while so you can deal with his friends. No healing in this set, but I found it wasn't missed.
Dark is a really great set, Tar Patch is basically a damage buff and an area slow, which works very well with high level robos. There's also a hold, and the totally excellent Fearsome Stare, which essentially turns a whole spawn into simpering idiots. The first power is a big area heal, but it has a long animation time. Oh, and you get yet another pet!
Other sets I don't know too much about, but it looks like poison isn't that hot. I could be wrong 
IN MEMORIUM OF GAYBABY
CRUELY TAKEN FROM THIS WORLD WHILE SO YOUNG
Newbie=New player
n00b=Idiot 
Also, one wrong opower choice isn't that fatal in Co(H/V)
Anyways, on MM sets... Necro/Dark for best survivability, Robo/Dark for best (especially AoE) damage, anything else for just the consept.
I'm getting CoV the weekend and I'm dead set on making a MM. However, since I'm relatively new to CoX as a whole (WoW Veteran for 1 year!), I could do with some advice.
So, if you please, a list of pros and cons to the MM primaries and secondaries. Normally I wouldn't ask, but in this game choosing even one wrong power can totally screw you up!