Using these 14 teams as our sample, let's look back and see how the Rays have compared with them over time.  For all we know, maybe the majority of expansion teams suffer from low attendance for their first 10 seasons, until they've been in the area long enough to develop a fan base.  Have there been any trends in the Att+ scores for these teams over time?  Let's find out...

Overall Trends

First, the Rays have been around for 12 seasons, so here are the Att+ scores for all the expansion teams during their first 12 seasons (click link for larger view):

Initial Attendance for Expansion Franchises

Sadly there's no easy to notice trends within the data.  Some teams start off with great attendances and decline, some start off with below-average attendances but end up above-average, and with others there is no discernable trend.  That graph is a really poor way to try and view trends, though (waaay too cluttered and confusing), so here's the average Att+ of all the expansion teams during their first 12 seasons:

  First 12 Seasons - Average

While there is a slight upward trend in attendance over this time period, the scores vary from year-to-year and the change is so small, it could be a result of random variation.  At the most, it shows that teams have averaged a 5% increase in attendance over their initial 12 seasons.  However, the most important point to draw from this graph is that there is no set pattern to how expansion teams typically draw.  While some teams are like the Rays and start off with low attendances for an extended period of time, there are teams that don't follow that pattern at all.  Some teams, like the Rockies, experience incredible success right out of the gate and other teams, like the Rangers, suffer for an extended period of time, but neither result can be called "typical".

Time Periods

While there aren't any trends in the data when considering all expansion teams, is it possible there are different trends depending upon the time period?  Maybe comparing the Rays and Diamondbacks to a team created in the early 1960s is misleading, and teams from similar time periods experience similar trends in Att+ over their first 12 seasons.

Initial 12 Seasons Per Time Period

Woh!  So that's certainly interesting.  Teams formed in the 1990s averaged a much higher Att+ but ended up with a below-average Att+ over the course of 12 seasons, while teams formed in the 1960s started off slow but picked up steam as time went along.  The curve for 1977 seems to be a conglomeration of the two trends (start strong, decline, and then pick up steam), although there are only two teams in that sample.

Conclusion

So there aren't any sweeping conclusions we can make about expansion teams in general, but there are differences in how fans have responded to expansion franchises during different time periods.  It's tough to say why that's the case, though; maybe the earlier cities were larger and/or more receptive to baseball.  My guess is that it's because baseball didn't have stiff competition from other sports in the 1960s.  Football had just started gaining popularity and the NBA was relatively young as well, so baseball was still the "American pastime" and fans quickly formed connections to new franchises.  By the time the 1990s rolled around, though, baseball was on even footing with football and basketball in terms of popularity, and they also had a nasty strike that cratered popularity.  New expansion baseball teams had other sports teams to compete with for fan allegiance, so after their initial glitter wore away, interest and attendance faded quickly.

Right now, forming a new fan base has never been tougher.  Think about it: it's a rare fan that cares passionately for multiple sports teams.  You can root for multiple teams, but how many fans own season tickets to the Rays and the Buccs?  How many fans own season tickets to the Giants, Yankees, and Knicks?  We all have our biases: some fans care passionately about football, and only glance at the baseball standings every now and then.  For some, it's the other way around.  Because of this, I would imagine that the more teams there are in an area, the tougher it is for a new franchise to develop a fan base.  Passions are already in place and it takes time to change those, although how much time I don't know.  Back in the 1960s, baseball was the sport to care about, so it made it easy for those teams to develop their fan bases.  These days, though, no matter where you were to put a new franchise, I bet they would struggle to develop a consistent, large fan base.  It's a different world and too many fans have already sworn their allegiances to other teams.