Reasons why it sucked:
1) No consistancy in Animation. Balmung's sword size, in example.
2) Lack of explaining/substance in plot. Hardly anything explained... ever.
3) Had Monster of Day complete with music for Shugo's Data Drain
4) Every episode was on same pattern; Intro -> Problem/Focus/Balmung's Event of the Day -> Characters take Action -> Data Drain -> Finish. Was predictable and boring.
5) Including w/every episode was Shugo loosing tooth. While most people say this was cute, it was obnoxious after... the 1203918249284th time. u.u
6) Characters were extremely 2D. Hotaru was just kind, Shugo was an idiot, Rena was "^_^"ish, Balmung was "T_T"ish, Reki made NO SENSE at all, Sanjuro was... Sanjuro, etc. Never any substance about characters learned. While anime with entire episode dedicated to character background can be annoying [ie: Seta Soujirou in Ruroni Kenshin], it help flesh out character.
7) Way to cliche for me. There was part where the entire gang splits up near the end before going to "final battle" then regroups 1 by 1. Reminded me of Sailor Moon, the entire Digimon Arc where the cast is separated, the Yuyu Hakusho Movie, and FF7 when everyone returns to Highwind.

Edit: hmm... Pizza.