Looking at eveyone's abilities it appears all tanks get some type of AoE snap.
Pretty much - even if a tank doesn't have many AOE spells/CAs they can aquire AOE attacks (fired off auto attacks) through spending the AAs - i.e., crusaders have trample (agi line 4 4 8 ), warriors get dragoon's cyclone (agi line 4 4 8 ) - and of course there's always the hate transfers coming off other group members (swash and coer, etc., only for the encouter though not adds)

Melee sounds very doable and is definitely opening up more options for me, or did the latest update really hurt the ability for melee followers?
Yes it is totally doable - and a source of consistent DPS - not hurt at all really - just a bit of a pain at present but can be side stepped through a bit more extra jiggling

Either way.. what is the big thing about defiler?
Defiler buff more hp than mystics, plus have a pretty tasty attack speed debuff - but - for a pally lead group I still prefer the mystic (remembering I am biased towards 'good' characters - not necessarily the 'best' characters).

...a monk...
There are a few ppl on here that have a brawler in their 6 mans who would be best to answer this for you. Personally, I feel you would give up more utility by including a brawler in a boxed group - but that's my own opinion. From memory, the monk gets a few special benefits when it's close to death (speed and dps inc???) but if your tank is tanking, the chance of that occurring are slim. Also, it won't be taunting much (except maybe enough to stay 2nd on the threat meter perhaps). Plus, from memory, brawlers don't seem to have many mob debuffs (correct me if I am totally mistaken) so you'd be giving up the debuffs which another class can offer.

I want a solid tank, any of the 4
Well that opens up some more choices.

Guard + Def + Dirge + Coer
Guard + Def + Dirge/Troub + Brig/Lock
Guard + Def + Brig/Lock + Coer

You'd be pretty hard pressed dying with a Guard + Def + Dirge + Coer...