I've played half a dozen more games vs Necrons since I originally posted the 'Let's talk about: The Necrons' thread in March, so have had a chance to form a better opinion of their strengths and weaknesses. Link:
viewtopic.php?f=4&t=20466 The Pylon's AA is AA4+ TK(1). This means that it exceptionally good in some circumstances and exceptionally poor (per point spent) in others. Against heavily armoured fliers like Thunderhawks it slices through reinforced armour like butter, against swarms of cheap Ork Fighter Bombers it has a 50% chance of killing one.
If the pylon is to remain the sole source of AA firepower, I would recommend normalising it to where its performance is less based on what army the opponent is playing.
EG Pylon AA4+ TK(1) or burst fire 3xAA5+
I don't think moving pylons from the war engine allowance is a great idea, because although their AA would be more accessible elsewhere they are also TK firing war engines with living metal. Having AA from somewhere other than the WE allowance would be good, but having AA and TK elsewhere could be unbalancing. (That might be a problem inherent in having a single source of AA in a list that is also armed like a Shadowsword against ground units.)
From discussion in that thread it seems Destroyers at -25pts discount would be more palatable.
The C'tan should be a walker.
I personally think that any formation coming through a Necron Monolith that is broken should take a BM for its troubles (sorry necron players). At the moment it is not just the case that breaking Monoliths is sometimes a tactically bad idea because they get to teleport out and appear next turn in a vulnerable or strategic place, but also when a monolith breaks it gets a free double move. Because the Monolith is a skimmer and fearless, breaking one can cause it move behind enemy lines within 5cm of an opponent's formation, setting up the next charge from within.
I can't think of anything that gains more from being broken, so a BM on a formation that then uses the portal seems a small price to pay.