While your players need to be dedicated to progression, you need to be pretty dedicated to your members. Sometimes your guildies need some gentle encouragement, and sometimes they need a kick in the pants. Be sure to make your expectations clear to them and provide them with resources to improve their performance. It helps when you run instances with them for upgrades and to gauge their abilities.
To get people on schedule, mark your guild calendar with set runs and stick to them. Even if you have to pug out some or most of the spots, your guildies will learn your raid times and make their best efforts to be there. You can't control people schedules (jobs, families, school, etc come first), but you can give them a stable expectation for their WoW time.
My guild mixes it up with another guild of approximately the same size and same level of progression. We've been running our own 10-man Naxx, OS, and Vault raids, but draw from both for 25 mans. Our heavy hitters are taking on Maly 10 and Ulduar 10 while we work on getting folks geared for the next level of progress. You'd be surprised by how many guilds are in the same circumstance.
If you choose to do this, be fair when it comes to gear and assignments with your counterparts'. Make your expectations clear with both guilds and when you all are together, treat them like your own. It really helps to stay in regular contact with the other guild leader.
I have to admit that this has it's drawbacks. Sometimes it still feels like "us versus them," and neither of our guilds are willing to merge into the other. As the overall raid leader, I can get frustrated trying to keep track of everyone's schedule. Sometimes I feel like I can't communicate well enough with members of the other guild. It's not a perfect, long-term solution, but it does keep both of our guilds active in progression while we support growth in our own members.
As for not being able to tank, that takes flexibility. Dual specs have really opened up the tanking pool, and given tanks DPS and healing options when tank spots are full. It makes sense that you can't roll on tank gear until the established tanks have gotten their fill. That being said, once the tanks are geared most guilds will pass off to off specs before they disenchant. If you're stuck in a role that you don't really want, work it, be the best you can be at it, and build the set of your dreams with spare parts.
There is also the option of finding a guild that meets your needs. It does very little good to be a part of a raid guild that can't work together due to scheduling constraints or personality conflicts. You may be able to find a tanking spot that's just right for you if you peek around a little.
In sum, you have four choices choices:
1. Be patient and work with your own guild.
2. Be patient and work with another guild.
3. Run as DPS and build a tank set.
4. Find a guild that needs you as much as you need them.
Whichever you decide, best of luck to you.
For love, for honor, for pony,