Gas grills for outdoor cooking are becoming ever more popular. Are you looking to buy a gas grill? If so, keep in mind all gas grills are not created equal. As with any appliance you should do your homework before running out to buy one. One tactic you can employ is looking at the most expensive gas grills and pay close attention to how they are constructed and what's on the inside. Then, look at the cheaper ones and look for similarities.

I'll move from the exterior to the interior of the grill.

The Exterior:

Stainless Steel Finish - Just because is shines and it's made of stainless steel doesn't make it rust-proof. Let me explain. Stainless steel comes in different grades; usually with grills, it's a 304 grade. This is a rust resistant finish and will rust. Follow your care guide that you get with the grill; it will help you keep the finish looking good. The same goes for the fasteners that hold the grill together. If they're plain steel they'll rust. 
Note: If you buy a grill with steel fasteners you may want to replace them with stainless steel fasteners from you local hardware store. The best time to do this is when you assemble the grill for the first time.

Painted grills are no different; they need to be cared for according to the instructions from the manufacturer. Too many times grills are tossed out simply because the owner refuses to do anything to make it last longer.

If your grill sits out in the weather get a cover and use it!

The overall construction:

Here's a thing that you can test by just pushing on one corner of the grill. If you push (with a little force), and, it sounds and feels like it's fixing to fall apart... it eventually will... and earlier than you want it to. To start choosing a grill this is a good first test. Open and close the doors and drawers. If it feels sturdy overall at least you know the frame is good.

Burners:

Burners come in a few different types of metals. Try to get "cast" stainless burners if you can; it is a thick metal. Sheet metal (thin metal) burners will have to be replaced because of burn-through. That's because they rust from the inside out and there is nothing you can do to prevent that.

One of the biggest problems I've seen with gas grills is getting the fire low enough to cook slowly.  BTU ratings are important. Don't only look for the "high"... you're not buying a hot rod! Look for the low capability. 

The more burners the better, this gives you more control when cooking different things.

Flame up:

This is a common problem with the older grills and the manufacturers are getting better at design. Once again... do some research since you can't test this where you purchase the grill. If you have a friend that has a grill that you've seen in action and you liked its minimal flaming you should consider that brand. See how the expensive grills handle flame up.

Grill:

The grill should be stainless if possible (a good grade) with porcelain enameled being the next choice. If it's cheap stainless stick with the porcelain. Look at the spacing of the rods; the closer the better. Also the heavier the material the better.
Note: If you think you may want to use a griddle ask if the model you're looking at can accept one. Weber and others have them available as an optional part.

Repair parts:

The better grill manufacturers will carry the parts you may need to do repairs. Ask the sales person and check their web site.

Warranty:

This is a good indication of the quality you'll be purchasing. Look at it close and compare once you choose a few grills you may buy.

In conclusion:

It all depends on what you expect out of a grill. If you're going to use it occasionally and don't really care about something "long-lasting" right now, just go cheap; it'll serve your purposes. If you want long life, you need to educate yourself before making the purchase.

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Read all you can, knowledge will prevent sorrow.

Be well and cook 'till it feels right!