We are on Thaddius at the moment and making progress with phase 2. We recently hit the 5 minute enrage barrier, and now "merely" have to make 40 people consistently execute the positioning rather than just the 6 survivors we had for the last 4 minutes.

We have an issue though; frequently we will suffer major losses / wipes at phase 1 when tank switches defy our planning in a seemingly non-intended way. 9/10 times, our tanks will successfully land on the other side by positioning close to the edge (as far as you can without war stomp knockback being a threat) and right between the two corners. Often we see tanks just not making it to the other side without them having made apparent mistakes. Some simply miss the other side while we sometimes have people fly in a nice arc, only to suddenly drop vertically down when they're halfway over.
We are clearly missing something when dealing with this phase but it appears to me that it is more of a bug than intended game design. It took us two extra nights on Maexxna to realize that a different position would make the LoS bug less frequent and I'm not interested in going through something similar to get this working smooth, since it is not quite certain that we will finish Naxxramas in time, before the expansion ruins any opportunity to experience the last fights in their true form.

I'd like to hear whether you have been seeing similar issues at the Thaddius wights and if anyone can provide advice to make the tank landings more reliable.

Sit your tanks in the middle of the platforms.

Your tanks are too close to the edge, basically. It's stupid, but it exists and for now, just work around it by going a bit more into the center. All your mana users should be at the back wall anyway, far out of range of the stomp/manaburn.

And we've found you can practically just wipe and run back if you lose more than 5 people too early. The loss of the extra buffranks is just too destructive to your damage output combined with the natural bit of lag the encounter makes you endure, might as well get a pvp death (no durability) from the other team's Charges.

And I'll tell you right now, once everyone knows the fight (which should be after the first night of succesful phase 2 experiences), start introducing heavy DKP adjustments for people that screw up with their Charge or platform-> Thaddius jump. Trust me, it saves you a whole lot of headaches if you get it into people's heads that they really shouldn't fuck up in this fight. If you thought C'thun was a good way to test all your 40 raiders' capabilities, meet Thaddius :)

The above is, of course, only for the really stupid things where you know people just didn't pay attention. If people disconnect too much, tell them to get rid of some mods and lower their graph settings to create a bigger comfortzone for their shitty PC's. The encounter really pushes your PC (and the server itself...) to the limit, so don't do attempts at 9 o' clock when half your end-game raiding population rolls around in Naxx either.

Yeah, it does sound like a positioning problem on the part of the tanks. Only way around it would be to eat a few wipes and tweak the tank positions until they get knocked over properly. You don't need the tanks right on the edge, 10 yards in is fine.

Other than that, once you get through P1 with all alive (and make the jump ;)) he's not really a whole lot to figure out. Do some dry runs to practice the switching, once that's set in stone flask and buff up to go in for the kill. Execution wise this fight isn't anywhere near the complexity of C'Thun, which was brought up as an example. It may very well be deemed harder, it's dependant on ace gear, consumables use, and there's absolutely no room for error. But in the end all you have to do in P2 is what you normally do on any boss fight, and watch that periodical debuff. And the entire fight is all about keeping everybody alive. Those 18-19 debuff / buff stacks compared to just 15-16 really makes or breaks the kill.