The one Lone shows comes closest to what we have. I think the following features are important (some already mentioned):
Made of sturdy plastic rather than wicker so it's easy to clean. Imagine you've got the carrier on your lap at the vet's and the cat is so scared it pees or pukes - would go all over you! We always put his favourite blanket over the base so it's nice and comfy.
Has fitted slots on top so it can be fitted securely into a car using the seat belt.
As catneuterer says, a well-made door made of metal rather than flimsy plastic. On ours there are attachments on the left and right, so you can either just open one side and leave the door on its hinges, or take the door off altogether. I like the fact that the door is meshed rather than solid clear plastic because I can stroke a paw or push Dreamies through.
A two-part carrier, with attachments in the middle (the one lone shows has this) so that if a cat is cowering in the back you can just take off the entire top section. This is what our vets always do rather than risking sticking their hands into a carrier with a defensive animal. I wouldn't call it a top-loader, though, more a top-unloader. It would be tricky to put your cat in the bottom half, then put the top on afterwards.
Ours just has a sturdy handle on top - I would prefer a shoulder carrying strap too.
Make sure your carrier is large enough for the cat to stretch out in comfort. The smallest ones are only really suitable for rabbits. Our lad is nearly 6 kilos (12 pounds), so he needs a pretty big one which is also sold as being for smaller dogs.
I like only having ventilation on the sides, because if a cat is nervous you can put a blanket over the top and then they can only see out of the front - so they only have to be wary in one direction.