Why I dislike religion
I understand religion is not a monolithic entity. What is called religion by a pundit of the canonical texts differs from that of a Naga sadhu in Kumbh Mela differs from that of an orthodox middle-class auntie. Religion is not alone in defying definition. You'll never get two people to agree on their definition of government either! But yet we all agree that government exists, and so does religion. And like obscenity, we know it when we see it. Being so, I can only criticise based on whatever I see religion as. Take offense only when appropriate.
Firstly, any belief that the principles by which the world of humans operates are controlled (to whatever extent) by a masculine, singular supernatural entity, i.e., God, falls by the wayside as one navigates the vagaries of life, career, family and most importantly, Bangalore traffic. Even if you attempt to hold on to more sophisticated ideas of the soul like the theories of Atman in Hinduism, you never get to see the proof of it or experience it yourself. Why a trans-generational soul-like concept should have moral implications is also not clear. As to religious explanations for various physical and biological phenomena I have one word: Please.
Secondly, in its social and political manifestations, religion both helps and harms humanity. There is an admirable amount of altruism, tolerance and affirmation of values that contribute to human welfare. But since man is a political animal, there is an equally long history of abuse and cruelty by some of these institutions.
One problem underlying both these aspects is the suboptimal way in which religious ideas have been evolving, for many centuries now I guess. Which is in stark contrast to another topic: Science. Again I almost never discuss science here (and I'm not particularly interested or good at it), but I am a very strong believer in the scientific method (sorry, the Wikipedia page was verbose). While the general principle of hypothesize-test-publish seems like commonsense, when this is rigorously applied in an institutionalized manner by 1000's of people in many countries over hundreds of years, I have only one word for it: See?
Going beyond the steps themselves, the institutionalization also creates a system of checks and balances, codes of conduct and critical histories. This can prevent abuse. This is similar to the role of various institutions in a liberal democracy. Religious organizations don't have anything approaching this (again, remember they're not a monolith).
Which is not to say science proceeds straight as an arrow. A brief acquaintance with the history of science and the lives of its famous exponents will reveal many follies and foibles. Results in social sciences and esp. economics are still looked at with suspicion and don't get the same respect as their counterparts in the "hard" sciences. When you are aware that a better theory could come along any time, a degree of humility is natural.
Thus if you are immersed in liberal democratic or scientific discourse, you are used to certain assumptions and methodologies. And you scratch your head really hard when someone tries to derive important precepts from a book of verse written in Arabic, Hebrew or Sanskrit eons ago. And any claim that they cannot be translated into layman or widely understood is insulting and self-serving. One measure of vitality is the amount of new disciplines spawned (i.e., new subjects opening up for study) and combinations of disciplines (i.e, new ways of synthesizing knowledge). Religion fails spectacularly at this.
There's a third aspect of religion, which is neither the sheer intellectualization of the first nor the social orientation of the second. Religion can be learnt and lived in the confines of the domestic and the personal. A convenient and hereditary assortment of ideas and rituals which come out of hibernation at specific seasons and functions - a kind of familial glue. I have two words for this: Mostly Harmless :) Harmless because it doesn't impact people beyond immediate family. "Mostly" because it might still gag the spirit of enquiry.
I myself look at religion in an extremely piecemeal fashion. While I disdain most of its artifacts and activities, I find parts of it intellectually and aesthetically appealing. Even organized religion has lessons for modern governance and probably served a useful/powerful purpose at some point. There are people who see a need to actively fight religion as it is an alternative and regressive power structure. I am not so convinced about that. I see more glaring problems in secular institutions as they are (Hm, I ought to think more about this).
At an extreme, I tend to "un-define" religion. That is, during the course of human history, certain ideas were studied, discussed and written about. They had moral, social and political implications (and vice-versa). Now you are free to choose from them what you like, and frame a narrative that best fits your world view. Thus it's perfectly possible to situate religious events and ideas historically, chart their evolution inside the course of the overall life of humanity - say astrology within astronomy, temples within engineering, mantras within literature, philosophy within the epics and so on - without agreeing to the wholesale appropriation, monopolization and categorization of these artifacts/activities by your friendly neighbourhood priest, big mutts or the BJP. This is similar to how I don't allow the notion of government or the nation-state frame my understanding of free speech, free trade or other liberal values.