I found two aspects of the era system extremely attractive. First, it's visually compelling. The different eras are represented by different architecture for the various buildings. So, as producer/designer Bisser Dyankov showed me around his modern era city, I could see the multi-colored “Cold War” era apartments and compare them colonial mansions or modern skyscrapers. The graphical engine has been revamped, so the buildings also look better than they did in previous Tropicos, but even if they didn't, the idea of different styles appeals. Just seeing where I'd have expanded over the course of a playthrough indicates that Tropico is recording an architectural history of my playthrough, which appeals greatly to the history nerd inside me.
I can see the demand that I find new cash crops and economic systems reframing how I viewed each new scenario in Tropico 5, forcing me to view my city and island from a different perspective. One of the biggest problems I had with Tropico 4 was that, once I figured out the best method of exploiting what needed to be exploited on my island, I'd just do that and let any other problem slide until the timer ran out. That seems like it'll be less effective in Tropico 5. That's partially due to the eras alone, but also due to the philosophical focus on making the “sandbox mode” more important in Tropico 5. I played the fourth game's campaign scenarios and ignored the sandbox mode almost entirely, so I'll be interested to see if Tropico 5's eras help motivate me to just start a random new city.
That's not the only area where Tropico 5 is changing from its previous incarnations. Although the developer and publisher were proud of Tropico 4, when they met with me they both indicated that they understood and acknowledged its reputation as not being very different from Tropico 3. Thus they wanted to do many things differently for the fifth iteration. The eras are part of that, but they're not the only components new to this title.
One of the big new developments is a much more robust military system. Whenever pirates or foreign powers attack, you'll have some basic tactical choices about which areas of your island to focus defenses and attacks on. This is combined with an exploration system, where you send troops out to see new parts of the island, which may also leave your holdings undefended. The developers stressed that they didn't want to turn Tropico into an RTS where combat tactics would win matches, but they did want to increase player control and complexity of battles.
I liked almost everything I heard about Tropico 5. Everything Dyankov mentioned came with an explanation of how it would add interesting choices to Tropico, instead of existing merely to exist. Beyond eras, military, and multiplayer, he also discussed a research system, improvements to how factions were handled, and the addition of members of your Presidenté's family with their own abilities. But as effective as this philosophy sounds, I didn't get the chance to play it in-depth myself, so the big question question is how well these ideas will be implemented by the time it's released (For example, a question about improvements to worker AI, one of my big issues with Tropico 4, wasn't specifically answered). Still, the ideas are strong enough that Tropico 5 is a city-builder worth getting excited about.