Getting into this one late, working in order.
1) Skaven are a blast! Easy mode wins and near-impossible challenges, boom-boom-boom-boom-boom. If you're a block-monster like me, they will annoy you hardcore. They don't foul well, and they can't really hold their own in a punch-fest over time, even if you (the coach) have way better skillz than your opponent. Elves and Norse think they have this problem, but elves can dodge and can angle up really good surgical blocking, while Norse can hit back across the board. Still, developed Stormvermin can hit with the best of them, and Gutter Runners are great for mobile assists, so you can win a few fights if you can keep picking the battles.
2) Rat Ogre is optional but excellent, especially in leagues. Skaven are almost a different team with one. Sure AV8 bigs go out fast, but so does everything on the roster: it's not like CDs or Chaos, where you're balancing aggro vs tough. In a league, a RO is an alternative to a lino plus Fezglitch, it's all high-risk aggro. Having that ST at the point of attack is huge: give him Break Tackle and Guard as fast as you can. Don't blitz with him a lot unless it's just too juicy: there's a real art to positioning a Wild Animal so he can take 4+ Move actions without screwing you. RO makes one-turning super easy, btw. Second block on Frenzy drives into a SS GR, and it's off to the races. At that point even a pow is okay because the GR is across scrimmage next to another D-lineman, so it's just a diagonal block with another guy. Easiest chain-score there is with a rookie team.
3) Preferred builds: here's
My starting build (all formats). At low value Apoth is for any KO or BH, let anything worse go. You might forgo a KO Apoth if it doesn't happen in the first 3-4 turns, esp if you have any reserves. If you go into halftime with your Apoth, you either got really lucky or you're doing it wrong (at low value, anyway: later on you can save it for stars). In TV-based formats I forgo the RO and eventually go for 4 GR, 2 Blitzers, 6 Linerats. One GR is a sacker, one is a retriever/carrier/passer, two are cogs/outlets with Block/SS. Stormvermin are all kill, Guard is for Linerats. There are 3 normal first skills for linerats. #1 gets Kick, #2 gets Dirty Player, everyone else gets Wrestle. Doubles is Guard, follow with Block or Wrestle. Doubles after Kick or DP is Dodge for spam power. Second normals after Kick is DP, second normal after DP is Wrestle (or Kick if you need a new one). In a league I try to get a Rat Ogre early, because Guard is precious and Frenzy + Skaven = evil. If you find yourself stuck at/below 11, a Thrower is a decent hire in place of the fourth GR*, shave a little TV when he hits Leader at 6 SPP.
4) Political themes, it depends on your bent. I went through a phase of making fun teams themed around my contempt for a certain American
political party. I have a hard time not wearing it on my sleeve and still being funny: that's not an easy task. Since some of my best Blood Bowl friends are, well, you know, I decided to find other themes when I got tired of those teams and/or when they crashed. So you'll have to figure out where to go based on your own opinions.
5) There are so many great Skaven models. My favorites for pure character are Gaspez Topi. But my favorite of all is a kit-bashed squad of Mordheim or WFB conversions.
* I like 3 GRs on the pitch at a time. That means I like to carry 4 and 12+ men. In a league, I like 14 with a RO and a Thrower. In a matching format, 12 with neither, or 11 with a Thrower and only 3 GR.
What is Nuffle's view? Through a window, two-by-three. He peers through snake eyes.
What is Nuffle's lawn? Inches, squares, and tackle zones: Reddened blades of grass.
What is Nuffle's tree? Risk its trunk, space the branches. Touchdowns are its fruit.