I love myself some EDH. I've got about 5 EDH decks myself and am trying to build a couple more. Although, to be quite honest, only 2 or 3 of my decks are any good (including the ones I'm trying to build.)
Mana curve is still important, but it is definitely a slower format. The only exception is when you are playing against people with the expensive mana ramp cards (like Mana Vault and Sol Ring.) However, the odds of hitting those cards are very low (1 in 99 in fact due to the fact that your general doesn't hit the library usually.) You should be looking at around 40 lands/mana sources usually. You don't need a very large number of big, game breaking spells but a few is always good. It is a lot more important to have a lot of utility cards/creatures.
Bennie Smith is probably a good source for more ideas. His EDH Primer is pretty good and in 3 parts. Part 1 Part 2
and Part 3
are quite useful to gauge what decks need and the thought process needed to build a deck. (Assuming it's for a multiplayer game, of course.)
For Sen Triplets, the two cards you may want to look at getting is Mycosynth Lattice and Celestial Dawn. This allows you to play off-coloured spells from your opponent's hands.