Steam is great for softening stubborn wrinkles, so ensure your iron has some sort of "steam blast" to make ironing easier.
The water reservoir holds the water that will be heated into steam. Ensure that it's see-through & has a large capacity.
Fabrics require ironing at varied temperatures. Too much heat may burn your clothes, so you'll want an iron with adjustable settings.
Get an iron that is heavy enough to smooth wrinkles, but light enough to hold. Lean towards the lightweight side as the heat will do most of the de-wrinkling.
High-end steamers can heat up their water in a minute or less. Mid-level ones can take up to five minutes or more.
The manual will have the watts & energy usage - it may need significant power to run. It's not a huge concern for average use, but is worth noting.
Most steamers include one heat setting due to the fact that it's difficult to damage clothing using a steamer, regardless of the fabric.
In addition to de-wrinkling, your steamer can be used in other ways - to clean beds, drapes & furniture. Germs are killed on contact.