The tanking shortage.

There are many reasons for the tanking shortage in World of Warcraft, but the biggest cause is that the difficulty level scales inversely for tanks (at the present). Tanking an heroic raid boss is easier than tanking a 5-man heroic dungeon, and has been for the duration of the Cataclysm expansion.

So how can SW:TOR keep tanking interesting as tanks progress to hunting bigger and bigger game? Firstly, raid encounter designers can keep tanks on their feet. This can be done through choreographed movement (running out of melee range for an ability, then back in vs. unchoreographed trying to keep bosses [with their AI-driven pathing] out of randomly spreading ground affects), by adding interesting adds, and by making boss fights multi-boss affairs. In World of Warcraft, many people consider Ulduar to be the pinnacle of raiding. It had 13 non-vehicle bosses, of which only 3 were single-mob encounters (and of those three, one included an add in its heroic mode). Furthermore, all of the fights in Ulduar required some degree of movement for tanks, which combined to make the tanking experience challenging and exhilarating. SW:TOR's encounter designers would do well to keep Ulduar in mind while doing their own designing.

Secondly, encounters need to get more difficult as a tank progresses. Small group content tends to include more "trash" mobs than single-boss encounters. Tanks are often required to wrangle multiple enemies, organize crowd control, utilize stuns, interrupts, and survival cooldowns, often on a single trash pull in small-group play. Looking back to World of Warcraft, there are many instances where tank error will lead to a wipe in heroic 5-man content, but the last time that a tank needed to be really on-point in a raid encounter was the final boss in the Trial of the Grand Crusader raid instance back in the Wrath of the Lich King expansion (there are some fights that are moderately challenging in Cataclysm, but the last fight that stressed me out as a tank was handling the adds in ToGC). SW:TOR needs to make sure that they balance small-group content to be easier than large group, and also to utilize the features that can make small-group content challenging on a grander scale in their large-group settings.

Lastly, tanking gear needs to be interesting. From Burning Crusade through Wrath of the Lich King, encounters were tuned in a way that allowed (or forced) tanks to gear for threat (after reaching the block cap, if one was a Tankadin). In Cataclysm, between how hard bosses hit and reforging, the accepted best practice for tank gearing is to stack avoidance and mitigation and ignore everything else (Death Knight tanks often gear for hit/exp caps, but all other tanks are not afforded that luxury). Juggling gear to stay at or around the hit and expertise caps is interesting and fun, ignoring those stats at the expense of survival is not, especially when it leads to spikey threat generation and a general loss of control over one's character (missing a big threat ability multiple times in a row is not fun, and making the DPS wait on a newly spawned add with a relatively tight dps requirement is annoying and potentially raid-wiping). As tanking gear gets better and better, it should not lead to the gear becoming worse and worse for some content; as gear improves, it should become outright easier to complete previous tiers of PVE content, whether they are small-group or large-group.