I posted a thread similar to this the other day, however, what I dislike about them is not that there isn't much options, but some options really aren't worth taking.
Pariahs/Flayed Ones/Wraiths, they could all be a lot better written, Pariahs for example, far to expensive points wise to be considered effective for what they bring to the table. Not that I don't use those units, because I do, just not as much as I would like to simply because a lot of the times there really isn't much point.
I hate the Phase Out rule also, but maybe that is because it is such an obvious weakness in the army and everyone trys to exploit it straight away (Even me when I play against Crons so I don't know why I am moaning)
What I would like is.
New HQ choices, such as more customizable Lords (Lord with Wraith body) or I would love some of the characters in Necron fluff to be included such as the Herald of the storm/awakening.
Keep Immortals the way they are. Pairahs, either lower their points cost or make them more effective, I think it is there slow nature that makes them hard to use, a lot of the time when I have took them they are dead before they do anything useful, you can't even use them with the veil which is not good.
Keep the Warriors the way they are, I like them. I would say add another troops choice but for all my imagination I just can't think of one that would work and be fluffy?
I love Destroyers, I take them quite often so I wouldn't change them, Wraiths are hit and miss, some games they can be really effective for me, others they are terrible, but I suppose that is all luck of the dice and they way I use them but I know of other Necron players who have the same views as me on them.
The Monolith has got to stay as it is, I'd be upset if it wasn't.
Tomb Spyders I don't use much, don't really find them that effective as their points cost could be used more efficiently elsewhere.
Heavy Destroyers, I usually miss out, just taking the regular Destroyers instead but I do like these guys.
I would like to see another heavy support included, something like mini-mono or some sort of skimmer that resembles a Destroyer, but without the need for a necron piloting it, with a heavy gaus cannon on it, giving us a bigger punch and something with a better range maybe?
Or maybe a smaller version of the tomb stalker, that resembles a defiler but with a heavy gaus cannon or a new gun all together?