If you didn't get to see Naxxramas at 60 but did the first boss of the Mechanar in Burning Crusade, you're already familiar with the major "trick" to the Thaddius fight. Actually, you can make a pretty compelling case that Thaddius is easier than Mechano-Lord Capacitus, mostly because the platform on which Thaddius is situated is ideally configured for the encounter. Mechano-Lord was usually a mess of planning beforehand about where you'd go if you were melee and your charge was different from the tank's, etc. That's not a problem here. But before you get to Thaddius -- who, by the way, is the source of most of the eerie cries you'll hear in this quarter -- you have to deal with a few mobs along the way.
Unlike Grobbulus-to-Gluth, there is some trash between Gluth and Thaddius in the form of two flesh giants (who can be pulled one at a time) and (usually) a Shade. The Flesh Giants have a fairly nasty Thunder Clap-like ability that's also an AoE knockback, so you're best off pulling them back to Gluth's room and keeping them tanked well out of range of the potential Shade.
When you enter Thaddius' room, you'll note that it's impossible to get to Thaddius himself without jumping down from two platforms, one on either side of the entrance. On each platform is mob: Stalagg to the right, and Feugen to the left. They have to be tanked where they're standing; if they drift too far from the Tesla coils to which they're tethered, the coils will overload and wipe the raid with Chain Lightning.
You'll need to send a tank to each side and split your DPS and healers into roughly equal parties. The DPS split is particularly important because Stalagg and Feugen have to die within 5 seconds of each other. An interesting twist is that they'll periodically do an ability called Magnetic Pull which yanks each other's tank across the room (i.e. if Tank A is tanking Stalagg and Tank B is tanking Feugen, Magnetic Pull will leave Tank A tanking Feugen and Tank B tanking Stalagg). Each tank inherits the previous tank's threat on them, so it has no real bearing on how you do the fight, but it's fun to watch. Tank damage isn't particularly bad. Stalagg does more melee damage than Feugen, but Feugen augments his with some magic.
Your raid leader will need to /focus whichever mob he/she's not on in order to call out whether DPS needs to be increased or decreased on a particular side. Ideally you want Stalagg and Feugen down as quickly as possible because Feugen has a mana drain that could leave your casters and healers in bad shape, but ultimately it's more important that the mobs die together than that they die quickly.
After they're dead, there's an interval of about 15 seconds before Thaddius himself actually becomes active, so the next step is defeating The Ledge Boss. If you have problems with the jump (some people do, especially if they're lagging), a run speed enchant on your boots, or a run speed potion or ability is usually enough to ensure you'll jump correctly. After the Construct Quarter's been cleared, you can always come back and practice!
- Melee: Damage is respectable but not particularly difficult to keep healed.
- Chain Lightning: Every 15 seconds he'll hit up to 5 targets (8 on heroic) with this, doing about 3-4K damage on normal and 6-8K damage on heroic.
- Ball Lightning: This only happens if Thaddius has no players within melee range after being engaged.
- Polarity Shift: The only real "trick" to Thaddius but one that's deadly when raid members aren't watching their debuffs -- and everyone should have enemy cast bars enabled through the default UI or a mod. Every 30 seconds Thaddius will stop attacking and cast Polarity Shift, which is a very obvious 3-second cast. When the cast time is finished, each raid member will find themselves with a positive or negative charge. If you're close to players with the same charge, you'll gain a damage buff; if you're close to players with the opposite charge, you'll take damage, and rather a lot of it. Guilds typically designate the right and left sides of Thaddius for where to go for each charge; mine has everyone with a positive charge go to Thaddius' right, with negative charges on the left:
There is a very small 2 or 3 second grace period after each Polarity Shift where you can run from one side to the other (with the fastest way being to run straight through Thaddius himself) without "shocking" fellow players, but that's all you'll get. You have to move quickly.
On both normal and heroic modes, Thaddius will berserk after 6 minutes.
Tanks: Only one tank is needed on Thaddius, and he's not tough. Because you'll be getting positive or negative charges along with everyone else, you'll need to rotate him until he's facing the "correct" side. Especially if you're doing the achievement (see below), try to stand at maximum melee range to avoid accidentally shocking any melee players on the other side.
DPS: You'll see some crazy numbers on this fight due to the damage buffs you'll get from being grouped with similar charges, but more than anything else you have to watch that debuff. While Polarity Shift is charging, be ready to move instantly if your charge changes. As with the tank, melee players should stand at maximum melee range to reduce the likelihood of accidentally shocking players on the other side. As a note for raid leaders watching damage meters, ranged DPS is likely to do better damage on this fight because melee will often have no choice but to attack Thaddius from the front.
Healers: Tank damage is fairly steady on this fight but not generally difficult to heal. Chain Lightning also isn't too difficult to deal with, but it does come at 15-second intervals and is a little tougher to deal with on normal as it can hit up to half the raid at a time. As with DPS players, watching your debuffs is paramount. Make sure the tank is topped off (preferably with HoT's rolling) going into Polarity Shifts in case you get some bad luck and most or all of the heal team has to move.
Thaddius is the subject of Shocking!, which is the same on both 10-man and 25-man; complete the fight without anyone causing damage by crossing charges. This isn't a very difficult achievement, although I can tell you from personal and painful experience that any amount of lag in the raid or a disconnect is usually enough to eighty-six the attempt. Otherwise, it's most easily accomplished by people just being smart about their debuffs, moving immediately if your charge has shifted, and by moving back a little if your charge hasn't shifted. The latter will allow a little extra breathing room for people whose charge has shifted to get out of range quickly.
Killing Thaddius will also finish off the requirements for The Construct Quarter.
10-MAN vs 25-MAN
Stalagg, Feugen, and Thaddius are the same on both normal and heroic minus the usual differences to HP and damage.