Thankfully, all of my fears were removed in the gentlest manner possible.
Aria: the Natural continues the tale of Neo Venezia in very much the same manner as the original, except for the fact that this series is twice as long. The story, once more, revolves around Akari, Aika and Alice as they go through their daily lives in their "quest" to become undines. The episodic format of the original series is maintained throughout this one, and the show generally manages to achieve the same relaxed feeling of the original. However, Aria: The Natural has some moments that purists would define as "off-kilter" - a view I tend to disagree with.
Let me explain. The first series was very much an introductory show, allowing the audience to familiarise themselves with Akari, Aika and Alice, as well as the other residents of Neo Venezia. The Natural, on the other hand, is an introduction to the world of Aqua itself, something which was alluded to in the final episode of the first series. Indeed, the role of cats in this series is much greater than before, and the quasi-supernatural element of the show, which was only lightly touched upon previously, is very much in force here.
Because of this though, many people view The Natural as inferior to the original series, basing such judgement solely on the fact that this show has some genuinely creepy moments, as well as the fact that several episodes are based purely on supernatural events. In Aria: The Animation, the effect of the supernatural was toned down to maintain the relaxed feeling of the show, however The Natural pushes this aspect of Aqua firmly to the fore with the introduction of several elements - Cait Sith being the primary one.
Art and animation for The Natural remains as superlative as the original, with some improvement given to some of the more "fantastic" plot elements. Character designs remain unchanged though (which will come as a relief to many fans), with each of the leads and the supporting roles being as visually charming as ever - even in Super Deformed mode. The panoramic views of the Neo Venezia remain very much in force, however these have been enhanced more because of the attention given to other areas of Aqua. The level of detail is actually better in The Natural because of the show's focus on the world of Aqua, and with that attention to detail comes a degree of personality (yes, I know it sounds weird saying that a world has personality - watch the show and see for yourself).
Sound and music continue to be excellent, with the voice actors doing sterling work with their respective characters. The thematic music used throughout the series is very much in keeping with the style of the original series, and retains the same "continental" feeling as well. The sound effects are again, excellent and well choreographed, with nothing too overwhelming or too subtle. The OP and ED are calm and melodic, keeping the fluidly relaxed theme of both the original and this series.
The characters are, again, a joy to watch, however some people may not follow this train of thought because of the supposed "flaws" in the show (i.e. the supernatural elements I mentioned earlier). Akari, Aika and Alice continue to be as charming and as innocent as ever, and while there may be a distinct lack of major development in this series, I didn't find this factor to hinder my appreciation of the characters in any way.
The supporting characters continue to be by turns witty, soothing, stern, downright hilarious, etc, very much in keeping with the original series. However, many of the supporting cast tend to take more of a back seat in this series, with a greater amount of focus being given to Akari especially.
Fans of the original series may not find that The Natural sits too well with them, especially given the fact that this show is less about the characters and more about the world they live in. That said, it continues to be a truly enjoyable series overall, and a truly relaxing one at that.
Aria: The Natural, whilst maintaining everything that made the original series great, is actually a very different show, and I would judge it at least equal to The Animation in every aspect. Given the basis of The Natural, and its intorductory perspective on the world of Aqua, it's no surprise that fans recommend watching both series before watching Aria: The Origination.
