Couplers for high density polyethylene pipes need to be secure because you can't risk a leak of liquid or gas from the pipes they join. Even the best system will be subject to vibrations and general aging that could loosen couplers, so you need to get ones that really hold on and that let you respond to problems immediately. Look for these features, which can make joining pipes and keeping them together a lot easier and safer.
Just as a grippy interior can hold onto the pipes well, a grippy exterior will make it easier for you to get the coupler on in the first place. Couplers with slick surfaces make it harder to really push a tightly fitting coupler onto unyielding pipes; while you can use things like mallets to try to tap the couplers on, if you tap too hard, that will damage the coupler. It's better to get a coupler that has that rougher outside so you don't lose your grip trying to shove the coupler onto the pipe.
Once the coupler is on, the two joined pipe ends should work as if they were one pipe -- you hope. If something starts to go wrong, like the material flowing through the pipes is able to leak out a bit, or the ends start to warp or corrode, you won't know until the damage is quite significant. Plus, the material in the pipes could be leaking outside the coupler by that point.
To prevent delays in figuring out what's happening, get couplers that have clear middles. These really do exist; the ends tend to be opaque, but the center portion of the coupler gives you a nice view of the junction between the two pipes. That makes it easier to check on the state of the pipes during inspections, too.
The More Bolts, the Better
Some couplers screw onto pipes, and others let you clamp them down to form an airtight seal. The more clamping power, the better, especially if you want to prevent anything in the pipes from reaching the outside environment. Many couplers now come with multiple pairs of bolts that fit on both sides of the couplers. These bolts also allow for faster dismantling of the coupler if needed because you don't have to fight against any friction that's preventing the coupler from coming free (the friction is good because it helps keep the coupler on, of course, but that does make it harder to remove the coupler if needed).
Get the latest catalog from your coupler supplier and look at what other companies have to offer as well. The technology is continually improving, so you'll see better and better couplers as time goes on. Contact a company like Cabletec to learn more.