1
Grams of coffee in the capsule
This is the big one. Nespresso's own spec is 5 g for an espresso, 5.6 g for a ristretto and 5.8 g for a lungo. Most compatible brands that publish a number land between 5.0 and 5.5 g. Many don't publish anything, so I work it out from the box's net weight divided by the capsule count. Less coffee pushed through the same amount of water is exactly what 'tastes like water' means.
5.3 g or more is good. Under 5 g, or not published at all, is a yellow flag.
2
Aluminum vs. plastic
Ground coffee goes stale when oxygen gets in. Aluminum capsules are the reliable oxygen barrier; plastic ones can be fine if the barrier film is well designed, but a lot of cheap plastic pods aren't. Deal-site comment threads say the same thing over and over: the plastic pods are the ones that taste flat.
Prefer aluminum. If it's plastic, check how recent the reviews are.
3
Flow and machine fit
The most common failure isn't flavor, it's mechanics. If a pod doesn't pierce or seal properly, water channels around the coffee instead of through it: a slow, thin, under-extracted shot that tastes weak no matter how good the beans are. Capsules made to Nespresso's exact shape (Starbucks, L'OR, Peet's, illy) get the fewest complaints.
Search the reviews for 'slow', 'leak' and 'jam' before you buy a big box.
4
Freshness
Individually wrapped or nitrogen-flushed capsules hold up best. Watch out for clearance lots on Woot and third-party Amazon sellers: they're often cheap because the best-by date is close. Stale pods lose crema first, then flavor.
On clearance deals, check the best-by date before you celebrate the price.
5
Intensity numbers don't travel
Nespresso's intensity scale describes roast, body and bitterness, not caffeine, and other brands use their own scales. A '10' from a compatible brand can be milder than a Nespresso '8'. Compare intensity within a brand, never across brands.
Ignore the intensity number when comparing brands.
6
Brew it short
Every OriginalLine pod holds about 5–6 g of coffee. A café single shot uses about 9 g. Pull a compatible pod as a lungo and you're stretching very little coffee across a lot of water. If a pod tastes thin, try the espresso or ristretto button before you give up on it.
Espresso or ristretto size for compatibles; save lungo for pods actually labeled lungo.