The thing is even a given revision can be different in various production runs. In a nutshell:
Named revisions are:
1A, 1B, 1C, 1D, 1E and 2B. 1C is likely the 1D precursor and 1E a precursor to 2B, so you won't come across them.
1D has subrevisions: 1D.1, 1D.2 (very uncommon), 1D.3 (uncommon), 1D.1 -> 1D.4, 1D.4 (Commodore version), 1D.4 (Escom version)
2B also exists as a Commodore version and an Escom version.
Due to the series of 1D subrevisions, the timeline is most likely 1D.1 < 2B < 1D.4, so 2B is not necessarily more recent than 1D.4, there's some overlapping.
1A has full clockport headers (may interfere with some expansions) and "incorrect" resistor values for the audio circuit (1.5 kOhm instead of 680 Ohms)
1B is rather solid, one full clockport row instead of two (may still interfere), audio resistors corrected
1D.1 / 1D.4 are the most common, 1D.1 also exists factory upped to 1D.4 as already stated. A visible difference is "pure" 1D.4's don't have a link wire which is visible on 1A through 1D.1 boards. Upped 1D.4s do have it.
1D.4 Escom version usually lacks the RDY signal for the floppy drive, having replaced it with the CHNG signal. There's a link wire underneath between pins #2 and #34 of the floppy connector. Disconnecting it from pin #2 and resoldering it to pin #1 of the external floppy connector brings the board back to Commodore specs and allows genuine Amiga drives to be used correctly. The same applies to 2B Escom version, although strangely some are identical to the Commodore ones (no wire) and don't need any modification.
1D.4 has a detachable mouse port, so does 2B but in the form of a plug-in board.
Some 1D.4 boards have the partial clockport header on the left side of the row instead of the (correct) right side. Factory blunder, but can be corrected with some desoldering/soldering.
2B boards have solder spots for a header to easily connect an Amiga 500 keyboard, bypassing the handicapped A1200 keyboard controller that way.
Lisa chips have been produced by Commodore, HP and NCR. They're functionally identical but plugging an Indivision 1200 scandoubler/flickerfixer can be a hit and miss deal with respect to proper contact. Everyone has their own story with this, but generally the Commodore version is more likely to work "out of the box" without sanding the Indivision socket. The MK2 version is supposedly more tolerant.
Budgie has been produced in two versions, earlier boards may consistently have the -01 version, with later boards it's more or less random.
(If you want to get very geeky, even the plastic casing and the LEDs have undergone two revisions
)
Complete Escom systems (with the floppy-handicapped 1D.4 or 2B boards) also don't have a genuine Amiga floppy. After fixing the board, it must be replaced or converted for Amiga use.
Genuine Amiga floppies for the A1200 usually come from Chinon, Teac, Panasonic and maybe Epson. The keyboard comes exclusively from Mitsumi and the keys are identical to the A500 Mitsumi ones, so if perfect colour matching isn't an issue replacements may be obtained from A500 ones. In fact, the A1200 keyboard plastic base has a crossed-out "A500" engraving.
All revisions have a PCMCIA reset problem, there exist both hardware and software fixes.
All systems more or less function well on a basic level. When pushed by expansions etc problems may occur, this has gained the collective name "timing bugs". For a rather thorough treatment of the subject, consult
this page. Some history on the subject also
here.
Some other info and photos
here.
And some more info and photos
here.
You'll see more subtle differences on the photos: Modulator size, power filter shape, RAM chip size. Just look around.
Some ideas on floppy modifications (convert PC floppies for Amiga use)
here.
The AmiOracle section here is also a good source of general info.
This will keep you busy for a while
Just keep reading and you'll soon get past the "overwhelmed" stage. Any other question, ask away and someone will likely jump in.