It is all about the attendees! Period!
When you're submitting a talk, try to frame it in such a way that each attendee will get enough value out of your talk to justify the expense of them attending the conference (conference costs, travel, opportunity cost, etc). If all of the talks meet that criteria then you end up with a really awesome conference.
If you are talking about a technique or toolchain, make sure that attendees will be able to go back to their daily lives and implement what you talked about. More often than not that means the tools need to be readily available (bonus points for open source) and you need to provide enough information that what you did can be replicated. These kinds of talks are also a lot better if they are presented by the team that implemented the "thing" and not by the vendor providing the toolchain. For most tech conferences, the attendees are hands-on so hearing from the actual dev/ops teams that did the work is optimal.
Make sure you understand the target audience as well and make the talks generally applicable. For something like Velocity where the attendees are largely web dev/ops with a focus on scaling and performance, make sure your talk is broadly applicable to them. A talk on implementing low-level networking stacks will not work as well as a talk about how networking stack decisions and tuning impact higher-level applications for example.
What doesn't work?
- Product pitches (there are usually sponsored tracks and exhibit halls for that kind of thing)
- PR. This is not about getting you exposure, it is about educating the attendees.