Modern MTB - Spoilt for Choice

When I started mountain biking in 2008, the bicycle used to be simple. Apart from the riding discipline, there are not many complication in deciding what kind of bike you're going to buy or upgrade to.

For instance, it is either hardtail or full suspension (FS). And FS would most likely lean towards All-Mountain (AM) or Downhill (DH) with a few exception to XC.

The rest are quite standard.

Nowadays, true to the sport's nature (and industry), evolution and solution to specific needs have been churned out quite rapidly  thus giving a broad spectrum of customisation. And with that, limitations to a big extent. Hey, this is mountain biking, after all. You ride a rig that's suitable with you and your type of riding.

These are the choices (or limitation) that you could have in assembling your bike that I have thus far collected (except fat bikes, and the list may not be exhaustive):

• Wheel size: 26", 650B (aka 27.5") or 29".
• Headset/Headtube/Steerer tube: 1 1/8" top and bottom, 1 1/8" top and bottom tapered 1.5" or Giant's Overdrive2 (OD2) 1.25" top with 1.5" bottom.
• Bottom bracket: Square tapered, Shimano external, ISIS, SRAM GXP, BB30, PF30, BB86, BB92, BB79, BB386EVO.
• Rear Shifter: Matching SRAM shifter with RD, matching Shimano's shifter with RD (SRAM's Attack shifter can also do the job, but rare nowadays and limited to 9s).
• Drivetrain: 8/9/10 rear cassette and matching chain
• Drivetrain (11s): 11 rear cassette (XX1) with matching XD Drive hub, 11s shifter, 11s chain and single chainring (a really restricted grouppo).
• Gear combination: 1/2/3 chainring coupled with suitable cassette (see Drivetrain)
• Stem/Handlebar diameter: 'standard' Ø27.2 or oversize Ø31.8 (seems to be the standard nowadays, overtaking 27.2) and relatively newer Overdrive Ø35
• Stem/steerer tube interface: standard 1 1/8" or Overdrive 1.25" (related to: Headset/Headtube/Steerer tube above)
• Fork end: QR 9mm, TA Maxle (15mm/20mm/DH).
• Rear axle: QR 9mm, TA 10mm, DT Swiss/Maxle Lite 12mm (check that your rear fork end 'over locknut distance' is 142mm for through axle).

Phew!
Anything else I've missed out?