In the old days, interstellar colonization was pretty simple and straightforward (once you had a starship handy). Heinlein, naturally, provided the real estate pitch:
"Imagine a place like Earth, but sweeter than Terra ever was … forests aching to be cut, game that practically jumps into the stew pot. If you don't like settlements, you move on until you've got no neighbors, poke a seed in the ground, then jump back before it sprouts. No obnoxious insects. Practically no terrestrial diseases and no native diseases that like the flavor of our breed." (Starman Jones, Ballantine pback, p. 68.)Nova Terra, to be sure, was the pick of the lot. In the same book Heinlein alludes to harsh colony worlds – and later on, an Eden planet turns out to have non-prelapsarian locals already in possession, who intend to stay that way. But given a sky full of stars and a ship to get you there, why settle for the also-rans? Heinlein also supplied a host of secondary tropes, such as the utility of horses that can fuel themselves from a handy pasture and (given a stallion and a mare) manufacture their own replacements.
Unfortunately, as commenter Ian M. noted a couple of months back (in comments spinning off a post about the Moon), it is desperately unlikely to work out that way. Suppose a planet with complex life, and enough of it to have built up an oxygen-rich atmosphere. It may look like Paradise, or at any rate Earth. Convergent evolution might well produce para-forests and para-grasslands, just as dolphins have a similar configuration to fish. But dophins aren't fish, and alien life almost certainly will not be like us. Hydrocarbon life anywhere will be built out of the same basic building blocks, but with differing architectural details – and our digestive keys will not fit its nutritional locks.
The good news is that the local tigers and local germs won't find us tasty and nutritious. But by the same token we can't eat the local venison or berries, and chances are only slightly better that our cattle can graze on the grass. Plants have a far less demanding diet, and might well grow nicely in any soil that has nitrogen fixed in it. In fact they might grow too well, at least the ones that don't rely on bees or other terrestrial creatures as their dating service.
Terrestrial plants, devoid of natural enemies, might crowd the native stuff out of any remotely suitable environment – wrecking entire ecosystems. But this too could go both ways. To local para-algae we could be walking Petri dishes: warm, moist, and fertile. Our bodies' defenses, if any, are likely to take the form of allergic reactions, not terribly helpful to us.
In short, any garden worlds out there are probably not for us. Those valleys with forested slopes above babbling streams filled with flashing para-trout are the ultimate nature preserves, to be appreciated but not subdivided for housing tracts. Yes, theoretically we might simply wipe out the native life, then recolonize with a terrestrial ecosystem including ourselves. I don't think you have to be a Jain to find something repulsive about this.
Which leaves the option of terraforming. For every nature-park world we will probably find dozens that didn't quite make it. We do not yet know whether life arises wherever there is liquid water to be had – we may begin to find out on Mars and Europa. But if a planet has oceans but no life it is a candidate for terraforming, and only the ecopoetic or gardening stage is required – no need to sling comets from the outer system to provide water, or hoover up 90 bars of CO2 out of the atmosphere. Worlds with limited 'primitive' life may even allow a sort of biological nonaggression pact, the native forms going quietly on in their own local ecosystems. There are still ethical questions (we're precluding or at least greatly altering their evolutionary prospects), but not like the ethics of sterilizing a rich, living world.
Yet even interstellar colonization is not as simple as it used to be.
Related post: In the early days of this blog I commented on the largest-yet (still!) discovery report of extrasolar planets.