Its curious. When modified first hit, as Fulop and I were discussing earlier, nobody knew WHAT to do. There were no Modified qualifiers and the first real tournament to premier modified was the ECSTS. Deckbuilding became HUGE. The range of just strait up HORRIBLE decks compared to people who found the cream of the crop gave you a TREMENDOUS advantage in those days. Now, we have lots of tournaments to allow us to gague not only format shifts, but how each new set changes the metagame. This can be good and bad: it allows good players to metagame more accurately, but allows bad players a chance to netdeck a very up-to-date list.
As far as sheer deckbuilding was concerned its curious as well. Singles were a little less important because you didn't have Pidgeot/Magcargo to search them out of the deck. However that isn't to say you couldn't run lots of tech. With Cleffa, Elm and Wrath, you were able to cycle through a lot of your deck to get what you needed, and often the singles made the difference. While it seemed like Gatr was the best deck, a LOT of players still didn't run it (the majority, I believe) and there were quite big differences between lists. I find that nowadays people run very similar lists in most decks. Nidoqueen, Rock lock, BSP, Ludicolo, T2, ZRE, etc, lists between top players are often EXTREMELY similar. In fact I remember being very surprised at Vernola's GC Rock lock list last year, even though it was literally like 6-7 cards off mine. You'd be hard-pressed to find even the most cookie-cutter decks exactly the same at the STSs.
Playing skill is odd. I'd have to say it depends on the matchup. I liked the STS formats because it seemed like there were no autolosses. You were never a 70-80% underdog, maybe your WORST matchup was like a 40/60, but even then. You can have really good, skillful matches like Ludi vs. Nidoqueen or whathave you, but you still have the Rock Lock vs. Medicham blowouts. I hate that. You lose the baby factor, but I still feel that Cleffa was very good for the game, even if I cursed my 10th attempt to kill a banded baby.
I can't comment on 15/3, because I wasn't playing pokemon at that point (I had taken a break because of marching band and wrestling). I sucked pretty bad and was a total nub during the first WCSTS days (not much has changed =[ ), so I can't comment really there either. I did think the first modified was a great format. Jason's T4, T2, Hanley's back-to-back, Brook's back-to-back, Seena's T4 - 1st, my T8 - 1st have to be evidence that despite MASSIVE fields and best-of-one matches there was a ton of skill in the format.
My WCSTS report:
http://www.pojo.com/Features/WCSTS/sts1211/My STS Report (Matt Moss T8 day1 T1 day2).htm