Curious: if you were going to switch, how would you want to run this? Would you want to deploy it to your own EC2 instances, or would you want a hosted solution (just as SQS itself is?)
If you are greenfield and scared of hitching yourself to Amazon, why not go something like RabbitMQ? There is also RabbitMQ cloud providers as well.
HTTP based solution is easy to understand and implemented (if basic functionality is enough for your use case). Real MQs obviously have more complex protocols and not all client libraries are perfect.