Back to Basics Advice?
Sometimes we forget that what is common knowledge for some is completely unheard of for others. So, I suggest we revisit the question of what NXT kit supplements we most recommend---and were to find them.
My top priority would be short NXT wires, which are available from mindsensors.com. The regular wires get in the way excessively.
(While you are there, check out their new Touch Sensor Multiplexor.)
If I only owned the NXT 2.0 kit, I’d definitely purchase some gears and the rechargeable battery set. The battery pack is available from the LEGO website. Gears can be found in large quantities from LEGO Education, or in any quantity from Bricklink.com.
No matter which kit you have, more of these are desirable:
Technic Beams with Snaps
Studless Technic Beams
The links are for the Lego Education U.S. site. They can also be found on bricklink.com and, I assume, other regional/national LEGO Ed sites.
Other recommendations from y'all?
This post has been edited for accuracy by the author.
In various sets. And at LEGO Ed. They still seem to have pretty much all the gears available, just not in an "assortment pack" - you can still buy bulk 24, 20, 40, etc. toothed gears right from the website (as well as studded elements, studless elements, pins, speciality connectors, old treads, new treads, etc., etc.). The 12 and 20 toothed gears are also available from LEGO Retail right now in Pick-a-Brick (no telling how long they'll last, as the Pick-a-Brick assortment tends to rotate... and it's not available everywhere).
As to "other than Bricklink"... well, Bricklink actually works remarkably well. Especially for parts that are no longer available, from LEGO Ed or LEGO Retail or in any current sets. It's much more secure in many ways than, say, eBay, which many folks use.
If you want pieces... you find ways to get them. And those ways change, pretty much constantly.
This new battery pack (whenever it comes out) seems to have a higher capacity, although I'm not sure of the price yet... and, it's a DC system, which means it will work with more commonly available DC wall transformers (those I have laying about all over... the current generation of LEGO AC wall transformers are a much rarer breed over here).
Parts I like to see added? Well, for the NXT 1, I always add tracks. Since the NXT 2 already has those...
Gears mostly. And more of the hassenpins. :)
(the lack of which in the NXT 2.0 set I consider most unfortunate)
Yes, and certainly gears (the almost lack of which in the NXT 2.0 set I consider most disturbing and puzzling).
In Germany, a good web source for parts is the online shop of LEGO Education Germany.
However, the best source of parts in my opinion are LEGO® events - you'll find there a lot of vendors of (used and unused) parts of all sorts.