None. It comes with everything you need out of the box. This is one of the things that make this marker such a great buy. (although it would be nice if one day they came out with a board that would allow changing modes and settings very trigger pulls or via and LCD screen/scroll wheel)
Reliability, consistency, accuracy, board comes with plenty of modes.
It would've been nice to set board modes with trigger pulls.
I have used this marker for several months and have been very pleased overall.
To sum it up, it's the little things with this product that make it great. The design is extremely simple and, as an engineer myself, I appreciate that it is very well thought-out. Most people don't seem to care about the simplicity of their marker because they feel that they can learn to maintain anything (which may be the case for most). However, in the case of the Threshold, I believe that the very few number of parts it contains is what causes it to be a very reliable marker. It has not broken on me once, and I believe that is because there really isn't much to break.
Maintenance of this gun consists of unscrewing the back-cap, sliding the bolt out, and lubing 3 o-rings. Period. I have very little time on my hands, and I like knowing that I can clean out my gun after a day of paintball in no time flat.
The feedneck is great. I put the hopper on there, clamp it, and never have to worry.
The marker has great consistency. At first it would shoot +/-20, but after 3000+ rounds, it now shoots +/- 3. I did have a problem with first shot drop off at times, but using DOW 33 instead of the DOW 55 recommended by Dangerous Power helped that out.
The gun is precise. That is, I can land my 2nd, 3rd, and nth shot very close to the first. Also, I have never had a barrel break. I use Diablo Zen practice paint.
There are only two things that I would change in this marker. First, I would have liked to be able to set my firing mode (i.e. PSP, CFOA, NXL, etc.) via trigger pulls instead of having to open the grip and setting the dipswitches. Second, I have seen a video of someone firing their marker while using the stock barrel and then switching to a Dye UL barrel, and it is obvious that the Threshold is significantly quieter when using the Dye UL. This leads me to conclude that DP could have done a better job with the porting, although I feel that the quietness is fine the way it is, but some people may feel different.
Performance-wise, this is the Corvette C6 of paintball markers. It costs half or less of "top-of-the-line" markers but gives you equal performance. It feels really good spending $550 on something, letting strangers with their DMs and Egos shoot it, and get compliments on it all day.
If they would charge $1000 for this gun (and it would allow for trigger pull mode changes), it would be worth it. It is efficient and extremely reliable. Nothing will ever leak to ruin your day, and maintenance is fast. I really don't understand why anyone would buy anything else. Really, the only reason to buy anything else in my opinion is if you want that feeling of having something very highly regarded like a Dye or Planet Eclipse. But when it comes to the things that you can measure objectively, the DP Threshold won't let you down.