Ok, I've got everything released (I think) between 'Please, Please Me' and 'Let it Be'

But there was a ton of albums released after that - Beatles tapes, Eps, anthologies, box sets, etc. etc.

What I want to know is - what am I missing in all of those later releases? Are there more songs? Better versions of the old songs? Are they just remastered? What?

I'm a big Beatles fan, but I've never had the resources to buy all of those albums beyond 'Let it Be'

i think the versions you have are fine. unless you want new songs (using lennons demos) from the anthology set or want remeixes, you are fine. the only thing i would do is to get the past masters volume one and two. these contain the popular singles that are not on cd that were only released on single.
here is a sampling of some of the songs on the past masters set
Love Me Do (Single Version)
From Me To You
Thank You Girl
She Loves You
I'll Get You
I Want To Hold Your Hand
Day Tripper
We Can Work It Out
Paperback Writer
Rain
Lady Madonna
The Inner Light
Hey Jude
Revolution

Note: The Revolution on Past Masters is not the same as the one found on the White Album. The WA version is more of an acoustic version, while the PM version uses electric guitars.

I would also get the Red and Blue albums as well as all three Anthologies.

WHAT?! No no, if you've got the Beatle albums (from Please Please Me to Let It Be), you don't need the Red and Blue albums, 'cause they're just best-ofs. The three Anthologies are nice to have, and probably worth it if you're a big Beatles fan. Also, the BBC sessions are very enjoyable, so I think I'd recommend that. And yes, you do need the Past Masters vol. 1 and 2. It's all of their singles and b-sides (except the ones already on Magical Mystery Tour). These are all you will need once you have their other albums.

And the "Revolution" with the fuzz guitar is the single version that can be found on Past Masters vol. 2.

Yeah, you definitely need "Past Masters" vol 1 and 2 -- actually those were the first Beatles CDs I ever got, excellent collection of non-album singles.

Skip the Red & Blue CDs, they're duplicated elsewhere.

As for the Anthologies, you determine how hardcore you are -- I love them, but they're really for the total Beatlemaniac (except you do have the "new" tracks "Free as a Bird" and "Real Love" on them). Same with the Live at the BBC album. They're all a little pricey, 2 CD sets, but I picked them up on Ebay for less than $20 each, so look around.

since he already has all of the albums up to let it be, all he needs is the past masters set.

the electric guitar version is the one on the radio i think.