I used to think that the Space Marines were boring, and had no real personality. Now I know that I should have had more faith in those awesome guys at GW. Sure, the Space Marines do attract some players who just play them as vanilla figures, soldiers without a name or personality. Bah.
My realization of the coolness of Space Marines (I prefer "Adeptus Astartes") actually came when I was looking through an Inferno magazine. In it, a squad of Guard are rescued by the Space Marines. It finally occured to me exactly how much bigger Space Marines are than normal humans, and how much tougher and cooler. Plus, their power armour makes them look even bigger and badder. Boo ya!
Before I go any further, I have to mention that Marneus Calgar is a poofter. The Ultramarines in general are pretty fruity. I mean, come on, think of their names. "Hi, we're Ultra-Marines from Ultramar. Hyuck hyuck hyuck hyuck." Bah, I'd much rather be a Dark Angel, Salamander or Black Templar.
As long as I'm mentioning the Space Marines' bad points, the bikers and Land Speeders are fruity too. Who the heck rides into battle on a bike, with his guns on the handlebars? And I prefer to think of Space Marines as rock-hard warriors advancing menacingly on their enemies with bolters blazing, or holding against onrushing tides of Xenos, not flitting around like fruitcakes in Land Speeders.
But more than making up for two fruity units are the inherently cool units of the Space Marines, like the basic trooper and even more so the Terminators and Dreadnoughts. Nothing beats the armored might of an ancient and honored Dreadnoughts crunching over the mangled bodies of its victims as it mows down even more of the Emperor's enemies.
Space Marines are also cool for their very bad religious standing. They are the Emperor's chosen warriors, all with deep beliefs in the Emperor and his Primarchs. Their armor and weapons are inscribed with ancient runes, half-religious and half-superstitious. They say praise to the Machine God, who Xenos scum will tell you is some alien deceiver but all true sons of the Emperor know is the benevolent god of Mars and the Adeptus Mechanicus.
There are several small fluff things, not important alone, that add up to make the Space Marines really cool. One is that the Space Marines are survivors; they live for hundreds of years normally, thousands when encased in Dreadnoughts. They are very hard to kill, and even when totally incapacitated on the battlefield they can still be brought back to battle readiness. They know only battle, having been raised to fight since a very early age. And of course they have their ancient Chapter honors, which make them unique from any other Space Marines.
The Space Marines might occasionally be turned into faceless figures with fours for stats, but as any real 40K player will tell you they're anything but vanilla.